Hugh,
If this is a pre-loaded Vista PC then I don't think you can do Slodave's suggestion, IIRC, IE7 is tied in tight to Vista, I think the only way you can repair IE7 is with the Vista CD (smart)
The problem may be a add-on, try to start IE7 with no add ons
Start> All programs> accessories> system tools> Internet Explorer (No Add-ons)
I run FF and have not run into any problem's, you don't need IE to work to download/install FF
Here is a link to download the .exe install
http://mozilla2.mirrors.tds.net/pub/mozilla.org/firefox/releases/2.0.0.11/win32/en-US/Firefox%20Setup%202.0.0.11.exe
Let us know