upgraded system, now XP won't boot

yeah it is a driver issue with the storage controller probably. http://www.michaelstevenstech.com/moving_xp.html

ok, i'll get reading that, thanks.

I think that you'll end up re-installing XP... PITA, but at least you'll get a bug free install.

Nope, when you change the MB, the low-level hardware-specific info in the HAL (Hardware Abstraction Layer) needs to be changed to correspond to the components on the new MB. You'll need to do at least a "repair install" and maybe a complete reinstall.

I believe that when you change your system in a major way as you did Microsoft requires you to purchase a new licence or a new System disk unless you can talk them out of it. You are legally using the system disk on the same drive. This may or may not be the problem.

Huh????

This is only true if it is an OEM copy of XP. If it came with a storebought computer then this may be the case, but if he bought XP by itself from a store, it should be a retail version, and thus this wouldn't be an issue.

all you need to do is do a repair install, stick your xp cd in and boot off it, somewhere along the line choose repair install and select your copy of windows xp, it may look like it is re-installing but it isn't it is just going through and installing new drivers and adjusting the registry and everything again making sure it is fine. this should be all it need, however, note briefly that some things may not function correctly, but this is the better alternative than having to re-install windows, it also provides a simpler way of backing up data rather than doing it off your old system/mainboard.

Retail stores sell OEM versions. They are all governed by the same agreements as far as I know. Microsoft sets the rules.

You cannot just move a hard drive with an XP installion from PC to Pc, it simply won't work. Here is what you do after you install the drive in the new system.
1. Set bios to recognize the new drive as boot drive.
2. Set bios to boot to CD first, then hardrive.
3. Boot up with your windows CD, when you see the initial recovery console option, skip that and go on to the new installation.
4. Windows will detect that there is a previous version of windows XP on the system and ask if you wish to install new, or repair the old installation, this is what you want to do.
NOTE** Do not get confused here, when you first boot, you will be prompted to use the recovery console, that is not what you want to do. Be sure you get to the point where windows recognizes that you have an XP installation on the disk, and asks to install new or repair old install- repair old install is what you want to choose at this point. You will also see the warning on that page that says "if you choose new installation, all the files on "C" will be deleted," or something like that. BE very certain of what you are doing, if you choose "new installation" at this point, your drive will be completely erased of ALL information and data!
5. Windows will run through what looks like a complete new install, but it is not changing anything except deleting and replacing all the needed registry and drivers for the new motherboard.
6. You will have to reload all the second party drivers for your hardware after the install finishes and reboot, just like you did when you first bought the hardware. I recommend removing them and reinstalling from scratch to make sure they are installed and working correctly for the new XP repaired installation you just did. Although some things may seem to work correctly, they will most likely be buggy until you do this.
7. You will also have to do a complete Windows XP update redo, (if you allow XP to update it's self from the Microsoft Web Site) as any updates you have done to date will be undone.
8. All of your programs and files will not be altered or erased in anyway, they will work just as before.

You do not have to buy a new version of windows, but after you are done installing windows, it will have to be reactivated. Microsoft allows you to reinstall and reactivate Windows 4 or 5 times automatically over the internet. After that, windows will no longer reactivate itself over the internet. You will have to call Microsoft, explain to them that you upgraded your hardware, they will confirm that you have a valid copy of windows by giving them the serial number and CD-key from your XP package, and they will give you a new activation string over the phone. Once this is done, you get the 4 or 5 times again before you have to call again.

sorry, i meant i was using the hard disk with my original system (the old Mobo etc..) to prove that the hard disk itself was working.
i have 2 seperate systems (1 old and one brand new) and 1 hard disk, i wanted to use the old harddrive with the new system.

and thanks to everyone else who replied, i just found my XP disc so i'll try all your suggestions tonight, you've been really helpful.

You did get a lot of good advise. My contribution was very inaccurate as other posters pointed out. According to Computer Shopper magazine Windows XP users can transfer a license to a new PC an unlimited number of times, provided they stop using the OS on (and uninstall it from) the prior machine. Microsoft will not limit the number of times a Windows Vista license to a different computer. (January 2007 issue)