Before you install IE7...
Before installing IE7 make sure you have disabled your antivirus and antimalware protection on your computer. IE7's setup write a large number of registry keys. A common way anti-malware applications protect your computer is by preventing writes to certain registry keys used by IE. Any registry key write that fails during setup will cause setup to fail and rollback changes.
See for more detailed installation instructions this post: IE7 Installation and Anti-Malware Applications at the IEBlog.