Dell Inspirion 1525 HELP!!!!

Hey, i seem to have a very big problem with my laptop. I have down graded it from the Original Vista operating system to Windows XP. As i think it is a better operating system for the computer.

The problem im having is when ever i turn it on it will load my POST but wont boot into windows, i tried loading into safe mode but it crashes and resets and when i try to reformat the computer tells me It can not find any HardDisks installed when obviously i know there is .

Im guessing its a big nasty virus or something as my other deskptop got the same problem but i was able to reformat it, strange.

Any help with this would be greatly appreciated.

Added note: The laptop worked for 6 months previously on XP so the downgrade is not the cause of the problem.

Any questions please ask.