AV programs usually run scans in the background, and sometimes randomly stop applications to scan a file they are using (happened to me in BF1942 on my 1.2Ghz laptop.. not pretty
). Just disable all things saying "real-time protection" or "auto protect" or things like that, and things should run a bit mroe smoothly. Just be careful what you do without those features, and run manula scans often. And never disable your firewall.
As for Stuffit, try winrar
instead. I love it.