Computer won't turn on, even tho fan is on.

Try to clear the Cmos. Could be a motherboard or Power supply issue.

is it very old.

There are a couple of problems that will cause a computer not to boot these are: Corrupted BIOS, Corrupted Start up files, corrupted hard drive (containing startup files) or a broken motherboard.

Check for a faulty motherboard.
Remove every connection and component from your motherboard except for the power, front panel, CPU assembly, RAM and graphics card, switch it on.
If you have more than one stick of RAM test them one at a time.
If this works then reconnect your other devices (Switch it off first!) one by one until you find out which one is causing the problem.
If your hear sounds of the disk starting, but nothing ever happens on the screen, the BIOS might be corrupted. That can happen when you try to flash it with a newer version, or if an ugly virus flashes it with garbage. If so, the BIOS needs to be reloaded correctly, or the chip replaced. Note: Not all computers have their BIOS in a flashable EPROM, so this might or might not apply.
If the board makes a sequence of loud beeps, there is a hardware error in one of the basic components. Two beeps mean 'no monitor found', four beeps 'no memory found', etc. (that list is not for sure). It is the BIOS reporting that it misses an elementary part to continue. Find out which part is broken (or really missing), and replace it.

Replace the Boot Sector.
Place windows installer disc into drive.
When prompted, press R to start the recovery console.
When asked which system you want to log into press 1, then enter.
You should now have a C:\WINDOWS\ prompt, type FIXBOOT, then press enter.
Type FIXMBR and press enter (that means 'FIX the Master Boot Record' = FIXMBR).
Reboot the machine, if it works, you are done.

Check your BIOS settings
To enter the BIOS menu you must tap a certain key as soon as you switch on the PC, this is usually DEL or F2 but it will say on screen, e.g. 'PRESS DEL FOR SETUP'
All BIOS menus differ, what you are looking for is a bit called Primary Master, which should have some numbers after it, if it does, then your drive is not at fault, move on.
If Primary Master has no information after it your hard drive is not being detected, if you are able, change the setting next to Primary Master so that it says 'Auto'.
Save and exit the BIOS, if your computer boots, stop here.
If your hard drive is not detected this time you will need to open up the case and check the connections. http://www.wikihow.com/Hook-up-a-Hard-Drive
Fix a PC Which Won't Boot Step 7.jpg
7Check for a faulty hard drive
Remove your hard drive and put it in another PC if possible.
If it does not work in another PC then it is dead, buy a new one.
If it does work in another PC, try using different cables.
If different cables make no difference your motherboard may be faulty.

try another power supply first. a lot of units if they fail can fail with low output. if you have a volt meter check the outputs.