You'll need a program like VNC to do that. Just be sure to be cognizant of the security implications of using such programs. You could potentially be opening a very big door for someone to come walking through.
I'd suggest a compromise of you having to call this other person, and tell them to start the program, which doesn't run at startup. And I mean actually call, it's easy for someone to hijack an IM or email account and impersonate you. Much harder to do with a phone. It is a bit of an added hassle, but very little when you consider the security headaches it prevents.
I am running Windows Xp Home. I work on my relatives computer fairly often and would like to utilize Windows Remote update feature (her computer is XP Home also). Is there an option to make it so there is no expiration date and also that she doesn't have to select ok or whatever option it is? Basically I'd like to do the updating from my house whenever needed and not have to make sure she is there.