have you added any hardware recently. if not try system restore to return youre pc to a pre-problematic state.
how to use system restore =
click start type system restore
select a date before you had a problem
there are 3 dates but if the problem started before that date, check the box in the lower right hand corner for more dates, it may be a good idea to click on the box for scan for affected programs if this problem has been going on for a while.
click next and follow the prompts. the pc will reboot and attempt to return it to an previous state.