http://www.rickrogers.org/fixes.htm#Installation

Try cleaning up the installer. Click start/run, type services.msc and click “ok”. Scroll down to “Windows Installer” and double click it. Stop the service if it is running, set the startup type line to disabled. Click apply/ok and reboot.

Delete the contents of the temp folders, there may be a file in there interfering with the setup routine of the installer. Check both:

C:\Windows\temp C:\Documents and Settings\<username>\local settings\temp