Are one or more web sites not doing what they're supposed to be doing?
My knee-jerk reaction would be try another web browser. Internet Explorer users should install Firefox if for no other reason than this. Rather than debug a problem, it's easier to work around it when possible. If you've never used Firefox before, I suggest starting with the portable version. It's less intrusive.
Windows XP users with a mis-behaving instance of Internet Explorer, should read Microsoft's Knowledge Base article How to reinstall or repair Internet Explorer in Windows XP.
One of the suggestions for dealing with a broken copy of Internet Explorer version 6, is to upgrade to version 7. I agree with this. For a long time I avoided IE7 (not that I use IE much anyway) but at this point IE7 is mature enough to have had the most obvious bugs and design mistakes fixed.
If IE7 is problematic, the article suggests starting it (perhaps a big assumption), then navigating as such: Tools -> Internet Options -> Advanced tab -> Reset button. This opens a new "Reset Internet Explorer Settings" window, shown below. Click the Reset button.
IE6 diehards can re-install the browser with: Start button -> Run -> "%systemroot%\inf" (without the quotes). This will most likely take you to the C:\WINDOWS\inf folder. Right click on the Ie.inf file in this folder, select "Install" and restart Windows after the re-install has finished.
No matter what browser you use, Windows XP users should run it under the control of DropMyRights, a program I devoted three postings to back in August. To get started see.
Update. April 5, 2008: Today I tried using this on a broken copy of IE7 which crashed trying to display the default home page, every time it started up. I got IE7 running by changing the default home page to a blank page using the Internet Options in the Windows XP Control Panel. Then I ran the "reset" as described above. The next time IE7 started, it tried to display a "runonce" page at msn.com and crashed. Since the runonce page never finishes, it gets invoked every time IE7 starts, and crashes. Thus I took a copy of IE7 that could at least start up and broke it so that it no longer even does that.