I run a simple virus and malware setup combined with Adblock Plus on Firefox 10. Although there are times when the board is slow, it's not slow all the time and at it's worst (during prime time) it's still fast enough. I can expect 5 to 10 seconds waiting for a page to update sometimes. Then there are times when only a second or two is experienced.
Microsoft Security Essentials and Malwarebytes (the $24.95 lifetime paid version with real time protection) keep me clean and free of threats. I frequently get little messages in my taskbar that Malwarebytes blocked a re-direct to a rogue web site, not from ECF, but in the normal range of browsing in a particular day.
You might want to run Windows Defender to check for nasties. You can open it from the start icon by typing defender in the search box at the start button. I'd also recommend downloading Malwarebytes and running a scan on your computer ... that's free. Microsoft Security essentials is also free and heartily recommended by those who test antivirus software. Even if those don't find anything, at least you'll know that your computer is clean and you can concentrate on other reasons, like your ISP.