Hardware

You either need to try a repair install, that requires an XP CD of some sort, or possibly you need a new HDD and the XP CD of some sort.

The OS is corrupt and/or the hard drive is failing or has bad sectors, which has corrupted the OS.

If you download and install the Ultimate boot CD (UBCD) or Hirens boot CD, you can run some basic disk diags.

When I hear the words 'old Compaq' that sounds like a hard drive failure issue to me.

sometimes, files like drivers, become un-indexed and can't be found by the system. so you might want to try running a check disk on the hard drive in order to re-index the master file table.

Your computer is getting a Stop Error (aka Blue Screen of Death) every time it boots. By default Windows will immediately reboot the computer in case it's a one-off error. You need to see what the error is. To do so press F8 just before the Windows starts loading. From the menu that appears choose "Disable automatic reboot on failure". Now when you get the blue screen it will stay visible on screen and you can read it.

Reboot the computer three or four times, doing this each time. If the error message is the same it is probably the real problem. If it's something different each time you've most likely got a hardware problems, like bad RAM, hard drive, power supply or maybe motherboard/CPU. If this is the case I'd recommend buying an ex-corporate machine off Ebay to replace it. If you want to spend time though you can run chkdsk to check the drive. Power supplies can easily be swapped if you have access to a spare. RAM can either be swapped if you have some spare, or you can try one module at a time to see if one particular module is at fauly. If you get to motherboard/CPU then there's not a lot you can do.

If you get the same stop error each time let us know what it is, or do some Googling yourself.