I am running Windows 7 x64 (version 6.1.7601). My copy is also from the MSDN with latest updates applied. When I get to the end of the postgres install I get a error:
Problem running post-install step. Installation may not complete correctly.
The database cluster initialisation failed.
I very much doubt that s the issue being discussed here. I'd suggest starting a new thread on pgsql-bugs and including your installation logs from %TEMP%.