On a windows system, I wouldn't use anything but Vipre. Links on the main forum page (MT gets a commission if you buy through there)
I used to swear by AVG, but they did some stupid things that got me to switch to Vipre, which gave me a serious performance boost AND found stuff that AVG, Adaware and Spybot all missed.
This reminds me.
Make sure whatever antivirus you have is properly configured, your firewall is engaged, and you have spyware protection running. Vipre does all 3, but other software doesn't. Also, don't try to run 2 different antivirus programs, they can cause performance issues.
Browser wise, with Firefox:
- Make sure you've got the latest, but not a beta.
- Plug ins, Adblock Plus and NoScript. Disable everything else.
- Ditch any fancy 'toolbars'. Most are bloated, buggy and often spyware.
See if that speeds things up. You can reenable other plug ins as you go to see if that's the issue.
Also, if you are running GreeseMonkey for any reason, disable it. It really drags performance down at times, especially on Facebook.