gotomypc is easy, and it has a trial you can use. However, the person whos computer you are accessing has to let you install it on their PC too.
RDP (remote desktop protocol) is a great feature built into WinXP. I'd suggest using this. There are a few conditions to make it work. First the computer you are connecting to must have it enabled (Right click 'My Computer' go to properties > remote tab > check the box that says 'Allow computers to remotely connect to my computer'). Second, the person you are connecting to must have a password protected account on their computer. If there is a firewall make sure that the remote desktop is an exception. As in the case of the built in Windows firewall, theres a check box right int he exceptions tab to allow it. Now, when you connect the person's screen will get locked out, meaning they cannot see what you are doing. It's like locking your computer (Ctrl + Alt + Del - Lock Computer) - so all they see is a logon type screen. They cannot see what you are doing like I said, but you have complete access to everything on the machine, so long as there is an internet connection (which means if you restart the PC remotely you will lose connection). Now, RDP has some cool features, you can access shared files and printers, you can access their drives as well as yours (which is great for file transferring... yes you can transfer files from one PC to another remotely using RDP) and you can have access to their serial ports. There are other various settings that make your remote experience easier. I mean after all, you're connected via the internet so lag can be experiences, which is why they have a feature to turn down the settings, so you don't see themes, animations, images, etc. to make the connection quicker. It's easy to access too. Just go to Start > Run and type 'mstsc' and hit enter. It opens a dialog that asks for the computer you connect to (if you are on a network you can just type the name of the machine, or by IP). Make sure you have the right IP, and it's not a masked IP by a firewall or else you won't be able to connect.
There a little overview of the remote desktop feature built into WinXP