Remote desktop & VPN

OK to start off I am not very computer savy and this is why I need your help. I need to remote access into a clients computer and can't. I was able to with my old laptop (which crashed last week) so I know that there settings should be allright. However, using this computer I can get thru the VPN but when I try to connect to remote dektop after that I get the error that it won't let me. Can someone take me thru step by step what I can do to check to make sure all my settings are correct.