I had this come up today and I have no idea why. The article clipped is about Server 2003, but I found that the first fix mentioned worked for me on XP SP2:

1) go to the appropriate framework directory (probably C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727)

2) run aspnet_regiis -i -enable

In my case I ran it first as just "-i" and got an error. So I tried the "-enable" and found that it requires the "-i."

Then I ran it again as listed in the instructions (with both -i and -enable together) and it ran without any errors.

And everything works again