Microsoft have announced that they have discovered more security flaws in their Internet Explorer browser for Windows. One of these vulnerabilites simply causes the browser to crash, but some of the others could be used to gain complete control of a victim's computer.
These bugs affect versions 5.x and 6.x of Internet Explorer, but not the recent 7.0 Beta. So, needless to say, patch up as soon as they release the update (which should be 11th April), regardless of which browser you use. The reason behind this is that applications use the Internet Explorer framework (such as Outlook Express), even if you use an alternative browser for day-to-day browsing (like Firefox).
There's an interesting story about this exploit here.