Using Avira, Its really good. has the best detection and viro updating (IMHO). I havent used Avast much or the newer versions of AVG.
Avira has always scored very high in wild virus tests (last i check in 2008), even beating most paid for anti-v.
The free version has one major gap in that is doesn't scan email. so if you're not using Gmail, but are using a email client like Outlook, it will not scan the incomming emails.
AVG-free or Avast on the other hand do.
Also good to have anti-spy/malware cover as most dont cover those that well.
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Use C-cleaner to delete old temp files.
Also can see what you have running on startup (can also check in manage PC, services)
stop some things you've installing running in backround.
C-cleaner can also do minor fixes of registry.
defrag drive, and if still slow (and IF a virus or legit progamm is not chowing cpu cycles) its always good to do a clean sweep and format and re-install windows (and all the drivers

).