Windows Remote Assistance has always worked fine for me. I've used it Vista-to-Vista and Windows 7-to-Vista. The person you're helping just needs to send an invitation (by e-mail or IM) and you're in: it automatically works around firewalls at both ends with zero configuration. Gaining full control (including control over the dreaded UAC) is very easy, just requiring a simple confirmation from the remote party.
All in all it's a lot simpler than trying to use VNC and also typically comes ready installed at both ends so there's nothing to download and set up.
That sounds like a sweet trick. – Nathaniel – 2010-02-02T05:06:10.163