Windows XP boot failure

I was running Windows XP on a P4, 512 MB RAM, 200 GB hardrive computer (no company brand) I noticed last night that applications were running slow, having problems closing etc. I expected a virus problem, but NIS 2005 did not detect any viruses. This morning I attempted to boot the machine as usual, and I found that windows would not boot after repeated attempts in any mode. (last known good, normally, safe mode, etc...) I borrowed a friends windows XP cd, i dont have mine on hand, and attempted to repair the installation. However, when it tried to copy files to the hard drive it said that it could not detect it. Naturally, seeing this i assumed that the harddrive had gotten knocked around, bad power lead, etc. I made sure that the connecters were securely attached and booted again, repair etc, same problem. I believe that the problem lies with the configuration of using a PCI card to connect the hard drive to the motherboard. (I dont know why, something about windows not being able to handle larger harddrives, i did not install this Anyway, any suggestions on what i could do to get my machine back up and running again?
PS. Im at boarding school so unfortuatly the only drivers/ disks i have are those i can download or borrow!

Please recommend any solutions

thanks in advance
peter