If your on anything before IE7 upgrade immediately, your at sea in a ship with so many security holes its amazing its staying afloat.

If your on IE7 you can afford to leave it a few weeks or months, but not too long the security on IE8 is much better than IE7.

Having said that I run Google Chrome which hasn't been out long enough to be properly understood from a security point of view.

Even if you don't use IE directly, upgrade it as many programs use it to display html pages.

Simon G