Last week i posted the likely cause of Visual Studio Web Test buttons being disabled was running under Vista and not forcing the Administrator account.  Since then i have gotten email from a couple of folks indicating they are on XP and this is happening to them too....In following up with the development team, turns we have bug in the Web Tests that this can occur too. 

We have a fix in the Beta SP1 which you can get from here:

Visual Studio 2005 SP1 Beta Registration