|Try to obtain the recovery disks from the previous owner. Failing that, decide if you can order recovery cd's from hp or buy Vista (or Windows 7) and install it.|
"We only want to use the computer for gaming really... "
That machine won't play any modern games; probably would disappoint you playing a 6 year old game like the original Far Cry too.
Although the machine has a pci-e 16x slot, you'll run into problems attempting to upgrade to a gaming class video card.
1. Space is very limited and you'll probably need to look for a low profile video card.
2. The 220W power supply is too weak. Unfortunately, it is also a non standard psu and you won't be able to buy a better one without great difficulty.
3. None of my searches have found a better replacement power supply.
4. If you did find a psu and video card that would fit, the cost would be horrible. I'm guessing about $250 for a pc that cost $399 brand new.
These little computers were made for internet surfing, letter writing, and so forth. Gaming would need to be limited to stuff like Freecell.