Your post's headers (Microsoft Outlook Express 6.00.2800.1106) tell us that
you're running outdated and much less-secure versions of OE, IE, and Windows
XP. (The "less-secure" part means that your problem is most likely caused
by HIJACKWARE or worse!)
Backup your personal data, then do a format & clean install of Windows.
Please note that a Repair Install (AKA in-place upgrade) will NOT fix this!
After the clean install, you'll have the equivalent of a "new computer" so
take care of everything on the following page before otherwise connecting
the machine to the internet or a network and before using a USB key that
isn't brand-new or hasn't been freshly formatted:
5 steps to help protect your new computer before you go online
http://www.microsoft.com/protect/computer/advanced/xppc.mspx
Also see:
Steps To Help Prevent Spyware
http://www.microsoft.com/protect/computer/spyware/prevent.mspx
Rogue Security Software - Microsoft Security:
http://www.microsoft.com/protect/computer/viruses/rogue.mspx