Bios warning cpu1 fan failure

Look in your BIOS.

The first cpu is listed as Cpu0 and the second one as cpu1, doublecheck you have replaced the correct fan
Moto

If I understand you right you have a motherboard with two sockets for two separated CPUs? Every CPU fan connector has 4 pins and are thought to input PWM fans in (the forth pin is the PWM pin that make the fans able to automaticly change its speed due to what temperature the CPU core has). If you dont have 4 pin fans this wont work and the ordinary 3 pin fan will always spinn at maximum speed. Its impossible to connect a 3 pin fan wrong, so thats excluded from the error list. Look in BIOS and try to inactivate the PWM and its "fan controller" tool to force both fans to spinn at maximum spped. Something worth to mention is what cable you have connected to the CPU fan pins. I use Corsairs watercooling H60 CPU cooler and the first time I made the misstake to connect the pump head cable to the CPU fan pin and the CPU fan cable to the system power fan pin instead of the opposit (and right) way... Just make shure you have connected everything in the right way and look how many pins the cable and the CPU fan connector have. If it still dont work try to flash BIOS (Alt+F2 for Asus boards) to the latest version, if you have the choise to use the motherboard manufactors program thats included on the MBs DVD to flash BIOS that way, use that tool instead! In my case I tried many times to flash it inside BIOS with a USB stick but failed every time with a message "boot block in file is not valid", but when I searched in the Asus DVD I found a perfect tool that is made just for update and flash BIOS. When using that it suddenly worked!

Try to remove the second CPU and its RAMs to see if the problem is part of a conflict between the two physical units. Reset everything in BIOS and see what happens.

If this still dont solve your problem I suspect the problem is broken circuits in the motherboard to the fan pins. If you are able to try the fan on another PC with PWM at CPU fan connector, and it works on that PC, the foult is either the drivers for your BIOS or sadly a broken motherboard. Wish you all luck in the world!

Make sure you use the SAME kind of coolers and the SAME kind of fans to both CPUs! A PC cannot tell if you have one Dodge and one Lambourgini installed without first beeing programed to be able to see the differens. Dont sure but can imagen a possible conflict here.

/DarthTengil

^Irrelevant, Mobo doesn't care whats installed, just that something is,
although you are correct that both 'should' be the same to ensure the same cooling performance
Moto

Motopsychojdn: What more specific items and menys in BIOS do you think is handling stuff like this, more than the rather average items I have mention above? Have both fever and fibromyalgia right now so I cant think clear enough, but togheder I beileve we can help this guy a bit further.

I'm unfamiliar with dell's proprietary bios screens, but if Op works through and looks for somethings mentioning fanspeeds, then possibly a control method will be nearby, although not all bios's allow changing of fanspeeds,
hard to say without either being there or knowing/being familiar with the bios he has
Moto

this is far away possibility.try every points listed below.

1)Remove all heat sinks
2)clean them including processor and mother board, wipe every dust bunny.
3)check if the heat sinks are both same kind and install correctly in position and don't forget to apply thermal paste between processor and heat sink
4)check the heat sinks are sat in proper position if not tight the screws or push the pins if u don't know how then youtube is there.
5) And most importantly update your bios if possible.  

Hum, have checked out the spec for Dell 450 and (not to be rude) it must be at least 10 years old. It doesnt even have SATA interface! All modern motherboard use the same backbone to build BIOS even if it can look very different from case to case, but this thing is from around 1998-1999. Nothing wrong with that but you may do yourself a big favour to get nowdays component and build a new mid-end PC yourself, the big BUT when you buy a ready-built PC like Dell, Compaq and other stuff is that you became very locked and in the hands to the companies, and in some cases just cant do that or this change either if it is in BIOS or if it is a hardware-related item. BIOS is used to be very locked on those machines, just beacuse the manufactor dont want you to be able to fix things like this on your own and force you to hand it in for an expensive service check where you bouth the PC from.

EVERYBODY: Learn and build your own PC for your own sake!!!

No panic thus. We will try our best to make your computer to NOT show what is shown on the screen right now. First, get in contact with the one you bought it from and ask if he/she/it have experied former problems like the one you have. Try to ask Dell and the fan/heatzink manufactor what they think the problem is coused of.

Something every PC in this world shares is that you always will get into the BIOS by hitting Delete key after the computer sound "piip" after booting, on the screen it should text "preparing to enter Setup" or something like that. Well in BIOS you should have a number of pages you shift and go to by pressing the arrow keys left & ringt. There you should be able to find settings in "Boot/Power/On Board Setting meny" like "warning if CPU fan stops, Show logo screen when boot, Activate fast POST" etc.
By unmark/disable show logo when boot you will see the whole series of commands and information roling up on the screnn during boot up, hit Pause/Break key on the keyboard and you will be able to freeze and cath whats showing on screen so you have time to read it before it continues to boot up and load Windows. Sounds strange that you have to hit F1 and F11 to be able to boot up the PC, but that perhaps is Dells own solution?

Try to get into the BIOS and look what else is showing during boot up, and come back here later and write what settings you are aloud to change.

Good luck and dont give in!!!

Have you had any progress/solutions of what we have written as answers? How do your PC work now?

finally my PC stopped working.... it was not detecting hard desk from last one week and I had to hit restart couple of times to give it chance find out the hard disk.

But it is not botting now... got stuck on bios screen, probably wont be able to find hard disk.
I don't want to trash it, It is great machine with great performance and yeah off course i bought it very cheap  

Sounds like the HDD is starting to die or is dead. I would get a new HDD. Do you have a Dell recovery CD(s) or a system hard drive image?