There might be nothing physically wrong with your PC.
It could be software related.
What anti virus and antispyware do you use? Are they up to date? Have you run them recently?
Have you defragged recently?
How full is your hard drive?
Have you checked to see what programs are running in the background?
Do you do anything in the way of housekeeping on your PC?
Do the blue screens tell you anything as the codes they give can quite often give you an idea of whats wrong.
Have you tried repairing windows?