You're best bet is to use Remote Assistance from the Tools menu on the Help & Support area in XP. You will have to allow remote assistance on the user's machine and you will also have to have the proper permissions to use it but it works well. Remote Assistance is better for several reasons but most importantly you won't need to know the user's password to gain access. They will have to grant you access from their side.
But, if you don't want to use remote assistance, you can gain access to the user's session of the remote client by using the /console switch in Remote Desktop. The best way to do that is to run a command from the cli.
C:\mstsc.exe /V:COMPUTERNAME /CONSOLE