The ASCOM Platform depends on Microsoft technology that is included in Windows 7, but is not enabled by default. Here's what you need to do before installing the ASCOM Platform on Windows 7:
For more details on installing the ASCOM Platform, please see Tim Long's blog post The Definitive Guide to Installing ASCOM Platform 6.0 Service Pack 1.
Thanks to Rick Burke for this and to Tim Long for the expanded blog article