Whenever Windows starts getting screwed up there are certain steps you can take in order to help determine the problem.  I suggest that the first step you take be to run an antispyware software program.  Usually most people start having computer problems thanks to a buildup of spyware and adware in their PC.  I also suggest after all that to run a registry cleaner such as RegCure in order to get rid of any excess registry entries left behind by any malware or adware.  Spyware and adware can slow down your computer as well as pose a threat to your personal information.