XP 64, IIS 6 and ASP.NET
My current dev machine is running XP 64, which is a first for me. In the default setup IIS was not installed, so I went through Add/Remove Programs and installed it, which gave me IIS 6. This in turn has several tabs for ASP.NET, but try as you might none of these are what actually turns on ASP.NET and you just end up with mysterious 404s on a application enabled directory that's configured just like the working ASP.NET on your other machine.
Well, it turns out that ASP.NET (even though it shows up in the Properties tabs) is not installed by default and if you go to Web Service Extensions you won't see it there. So next, track down aspnet_regiis which is in the Framework directory and run
Then go back to IIS Manager -> Web Service Extensions where ASP.NET should now be an available extension. Enable it and finally ASP.NET works.