First you need to create a user on your desktop and the remote machine with the same username and password on both machines.
Make sure msvsmon.exe is installed on the remote computer. It should be in your Visual studio 2008 directory.
on the remote computer do a runas /user:debuguser c:\fullpath\msvsmon.exe
where debuguser is the user you created above
You should see a window popup saying "3/31/2009 12:22:56 PM Msvsmon started a new server named 'debuguser@TSIK2552100'. Waiting for new connections."
now on your desktop do a runas /user:debuguser "c:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\devenv.exe"
where debuguser is again the user you created above
the load your project and go to attach to process, enter debuguser@remotemachinename in the Qualifier box and you may need to select the show processes from all users check box. select your process and you should be able to debug.