system
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I wouldn’t go with full format yet… too much work to do, and maybe (most likely) nothing is wrong with rest of Windows components. Try repairing (uninstall and reinstall) of Internet Explorer and Outlook Express. I know, you maybe don’t have problems with IE, but those two comes together, so there is no way to uninstall just one.
Take a look in here on how to do that if you are using Windows XP:
http://support.microsoft.com/default.aspx?kbid=318378
or Windows 2000:
http://support.microsoft.com/kb/254623/EN-US/
or for some other OS versions:
http://www.bleepingcomputer.com/forums/tutorial71.html