Ok,
sounds like your PC might need serious medical attention!
My old laptop got to the point where it refused to shut down; it used to get stuck on the Win 98 shut down logo and sit there. It had a lot of dodgy things on it....namely from using *cough*kazaa*cough* (which I don't use anymore as it's illegal).
Have you run
McAfee's Free Stinger? If you anti-virus is infected, which is could be this will pick it up. Are you running IE or Firefox as your browser? IE is unstable; as I mentioned before. I had some dodgy spyware and other stuff etc. on my new PC which ate Adaware and caused start up problems. I also used Trojan Hunter (available free...you don't need to buy it seems to be an everlasting trial!).