I just spent some time debugging a customer's machine. When trying to install the .NET Framework, the install hangs because the IIS service can't stop when the asp.net files are being installed.
IIS is failing to stop because it can't stop it's dependent services.
Eventually after investigating, it turns out that if you have Zone Alarm installed (even if its turned off or disabled), it'll hang the install.
You need to uninstall Zone Alarm before installing the framework. For more information, check out: http://www.iisfaq.com/default.aspx?View=A335
Thanks to Alison for helping me debug this issue.