To clear the cache, go to your Tools menu, Options. A new tab will appear, on the left pane select Advanced. On the right at the top is a series of tabs (not webpage tabs, below that), General, Data Choices, etc. Select Network. In the middle is Cached Web Content, and there's a button on the far right, Clear Now - click it. That's all. No it won't clear your passwords.
Also in the Tools menu is Add-ons. Depending on how McAfee installed itself, you may have an extension or a plug-in (on the left pane again) labeled McAfee... I would try to disable it temporarily, just to see if it fixes the problem.
Another thing to try, that I don't know how to do, is to check McAfee for virus definitions updates. My thinking is it should do this automatically, but it never hurts to force it.