1. make restore point.
2. de-install all zillion anti-virus and anti-spyware.
3. clean all the crap from your computer with
ccleaner.
4. install any of the anti-virus software above.
5. keep tracking of installed software. Install just the things you really need. Even if you uninstall any software, there are still remains of it in the registry. Better use an advanced de-installer like
Revo uninstaller.
6. Use ccleaner and ad-aware from time to time.