It seems very odd, have you had any problems installing anything else? iTunes needs a working Windows Installer, but it's hard to think that would be missing in a new PC. If you open Windows Explorer and go to tools>>folder options>>file types, is MSI associated with Windows Installer Package? (That's what you do in XP, it could be different in Vista.)
It would interesting to see if you can install the stand alone Quicktime - that's the one that does not include iTunes.
http://www.apple.com/quicktime/download/win.html
Added:
A Quick Google suggests something else.
Are you certain that you downloaded the XP/Vista version of iTunes
Message was edited by: polydorus