Many of us use Windows XP in the observatory because of its simplicity and stability. The ASCOM Platform depends on Microsoft technology that is included in Windows 7 but was not included in Windows XP. This has caused confusion because the ASCOM Platform installer asks for Microsoft .NET Framework x.x, and Microsoft has made things confusing for "normal people" in their version control scheme. To make a long story short, here's what you need to do before installing the ASCOM Platform on WIndows XP:
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 Tim Long for this and to Tim Long for the expanded blog article