Could be several things. A little bit more information helps as well. Operating System, browser(IE/IE7/Firefox/something weird)...etc.
Has McSuckfee always been installed or did this problem just occur after installing it? Clearing your browser cache, cookies, might help too. Another step to take is spyware/malware removal. Spybot S&D and AdAware tend to be some of the better, and easiest to use, tools.
Also to note: I've used Symantec, GriSoft, Sophos, McAfee, and a great deal other anti-virus, firewall protection software and performance generally and most of them drops at some level. Symantec's latest(360) will be 'awesome' when it comes out. Can't wait.
If pressed, I use Symantec. I'm not happy about it. But at least I can blame Symantec when something happens to squeeze into my computer and crash it. It's a scape goat(1 word or 2?).