I have adaware and AVG free versions and have not slowed down noticeably although internet can be a little jerky if a site has a lot of banners and/or popups. Before buying anything you should think of doing the following
Run Defrag
Run Scandisk
are your virus tables up to date?
Do you download and uninstall lots of programs fron internet and/or Magazine CDs? If so you need software to safely clean out your registry.
If these do not fix things then there are free programs around that can diagnose problems.