Windows XP: Pc problems

Pc problems

A socket amd sempron 1.8g, 1 gig ram. windows xp pro w/sp3, mcafee anti-virus and zone alarm firewall. I am having some trouble with a boot problem. I keep getting an error when windows won't load up, "<windows root>\system 32\ntoskrnl.exe is missing or corrupt". I woke up yesterday and windows would not load, see error message above. I put in my windows install disk and finally got it running again. Woke up again this morning and it happened again, same error. I now have it running again. Any known reason while this particular file would get corrupted. I also can't get IE 7 to open up at all, I usually use firefox or seamonkey so it's not a real big deal but I still use IE for microsoft websites. Any help would be greatly appreciated.