As likely as not, a slowdown is being caused by an addon within IE.
From the menu, select Tools, Manage Addons. There will be a list of stuff that has been added to IE, not always with your consent. Any on the list that you don't use should be deleted or disabled. This won't affect the behaviour of IE other than stopping it slowing down.
The reason for slowdowns at the weekend is probably one addon trying to update itself. Particular culprits for this are Java and Flash plugins.
Also delete all toolbars, I haven't found one yet that is worth a candle. They ALL slow down your browsing.
Chrome and Firefox suffer much less than IE but they do suffer. It is always worth mucking out your browser addons as part of regular maintenance.