this machine had xp on it then i upgraded it to win7 and it ran perfectly for a few months then pow.
How did you do the upgrade?
A clean installation of Win7 with a reformat?
If so, there's a chance that you have a hidden partition from which Win7 boots. It has no drive letter and is ~100 MB in size.
I would try deleting all partitions using a small application called DelPart
. Create a bootable floppy or USB stick. Download DelPart and add it to the floppy/USB stick. Boot the computer from the floppy/USB stick and delete all existing partitions. (Follow the prompts, I don't remember the exact procedure.) It will remove EVERYTING from the hard disk and to avoid making a mistake, if you have more than one hard disk, disconnect all other hard disks when using DelPart.
When done, try reinstalling WinXP on an empty hard disk.