Antivirus programs are normally worse than an actual virus. Depending on your settings, every time a file is created/modified (think browser cache/extracting files) it is put in a queue to be scanned, which can use a lot of disk I/O and CPU time. Keep your OS/plugins up to date and don't open random exe's you find on the internet, and you should be fine. If you want to be really safe, just use something like Sandboxie to run your browser and you'll be fairly safe.