Things to consider...
Be in administrator mode during installation.
Make sure you have IE 6 installed.
Upgrade ur windows service pack
Disable anti-virus during installation.
Make sure windows components are installed.

If you have installed the beta version previously then I suggest u re-format ur OS since the beta version's uninstallation isn't clean.

If this problem still exists even after you've formatted ur machine, then i suggest u using ur friends CD/DVD rom to install. It's silly but that was one of my problems b4.