My Computer does not Boot the Other Day

Hi! I am having this problem for the last few days, it is like that my computer does not boot the other day. I mean that my windows Xp was having boot up trouble so i installed a new one. The other day it was not starting up i shutdown my computer properly but still it does not boot in safe mode neither normal in safe mode a few files appear to boot but then nothing continues, in normal mode the black screen keeps appearing not even boot screen is displayed i have not installed anything recently i suspect it is a virus but the antivirus does not pick anything i have tried and installed new harddisk but the problem came back after a few days. I have repair my windows daily from Cd it is so annoying.
and silly too. plz help me.