one of the innate problems with antivirus software is that they are configured for maximum protection.. this means they scan every file that is run, opened, copied, moved, and downloaded.. many users do not need this level of protection.. it will slow down any machine but of course some more than others.. I recommend you check options and perhaps uncheck scanning on 'run or opened' ect.. I keep mine set to scan all email attachments and any downloaded files.. with this setup I have been virus free for the longest time.. since I'm a bit paranoid I check for virus definition updates every day.. I always manually scan any files I download to be sure..
now, if after playing with options you still find your machine crawling perhaps you should completely uninstall the AV program to see if the problem persists.. if it does then there is something else causing this problem..
overall I'd recommend Norton Antivirus..