Hi, Paul,
As far as I can see, it's the same.
I just pulled up a 2005 solution, clicked on the top of solution explorer (the solution), right-clicked, and added an existing project. So make sure you have the top level solution hilighted first.
Also....there's a "chance" it could be your anti-virus software. Reason I say that is because I had this problem a year ago with 2003, and it was because of some script setting in McAfee. Just a shot in the dark.
Kevin
p.s. get some sleep! I've learned the hard way (well, still trying to learn) how unproductive I am when I'm sleep-deprived.