Remote desktop

I am unable to connect to my office pc from home. The office has xp pro and a linksys router that my office computers connect to and my wireless broadband internet connects to which is shared amoung the office. Anyway I want to connect to my pc from home and I am getting the message unable to connect pc may be busy….
I have been reading that I may need to forward a port on the router to allow access to my pc because I am using DHCP. Is this what I need to do and if I do what port number do I use and does this leave a big hole in my security?