IE7 thinks i'm a mac

When I try to go to Windows Update I get the following message:

This website is designed to work with Microsoft Windows operating systems only.
To find updates for Microsoft products that are designed for Macintosh operating systems, please visit http://www.microsoft.com/mac/.

What can I do to fix this? My system thinks I'm still running mozilla 4.o....I've read that maybe my header add-on is not right?

Any information would be helpful.

Thanks