some sugestions:
*it could be you have a memory problem.
Maybe one of your memory sticks is malfunctioning.
*your harddisk is completely full, you have to delete some files on it.
*your pc is infected with a virus.
*your registry is corrupted
you could do a regclean/repair
*there are somehardwareproblems. maybe some wrong drivers were installed?
check out control panel -> system -> hardware tag -> decive control (?) it's the above button,
and then you can see a yellow question mark in front of the hardware thats not right installed.
*look in software, for installed components that werent there before, like nvidia display drivers etc...
