The best way to do this without having to setup additional software (if both you and the person in need of assistance is on Windows XP) is to start with a chat in MSN Messenger.
From there, the user can go to Actions/Ask For Remote Assistance.
You accept and the thing loads. You can then ask to take control and the person can see what you're doing at the same time, and you can release control to have him/her try what you just did or show you their problem. You can also add voice to it if you have a microphone or chat, transfer files and more.
I use it all the time to show things to my mom and other friends that need my help. It works REALLY good, and you have nothing to setup on either end... And it uses RDC as the engine, so it's lightning fast and it does beat WinVNC by far for speed.
I'm doing tech support for companies in the state with MasterShakes, and we are used to remote control tools, I speak by experience.
The only remote control software that beats the speed of RDC so far, is Citrix. And for Citrix, you need a central server, and lots of money! I've heard VMWare is not bad either, but it takes a lot of cash too!
Other than that, I've used SMS Remote Control, TightVNC, WinVNC, Remote Desktop 32, Citrix, Tivoli and trust me, Microsoft's RDC beats them all for speed!
Plus, even if you're behind a firewall, you can share your drives and other resources through RDC... which many of the other programs can't do.
Anyway, hope that helped!
Cheers