There are a number of possibilities- virus activity/ corrupted drivers or windows being unwell.
I would be inclined to check the file system first- if you run system file check (SFC) you can make sure that the core files are ok.
to do this goto the start button, in the run box type cmd and then hit return.
At the prompt type sfc/scannow and then again hit reurn/enter.
you may or may not need the windows disk - you will be prompted if your pc does not have an i386 folder on the root drive.
To eliminate the possibility of virus activity install a copy of avast home edition (free for home use). Once installed allow it to scan at boot time.
For dodgy drivers go to start then run and type msconfig. then select "load basic drivers only" and reboot.
Post back your findings and we can take it from there