You'll need to check with Event Viewer, located in Control Panel, Administrative Tools. Windows should make a error log that should describe exactly what happened. Copy the text of the error message and enter it into a Google Search to see what others have come up with as a fix.
Alternative solution... have you tried Microsoft Fixit?
Another alternative... installing on top of another corrupt installation will usually fail in similiar fashion (seen this often with .NET and Java). Uninstall ITunes first before you install the new version, make sure you have backup (check with Apple... I do not use ITunes).