Have you tried re-downloading the Steam installer?
Checked for hidden files?
Formatted?
do you have multiple drives?
Have you had steam installed previously on any of your drives?
turn uac off
right click installer, run as admin...
worth a shot.
failing that, create a new user account and try there
Failing that....
format c:
You could always try valve tech support maybe, never used them but they might know? or google randomly and hope for the best?