I used to use Dameware a lot at the last place I worked. It's a pretty nifty utility, it will force the client software to install on the remote computer provided that you have the proper credentials, username and password. It's basically a VNC program on steroids.
http://www.dameware.com/
But I'm pretty sure this isn't a free program, I was using a cracked version because the company I was working for was too cheap to buy it.
edit: Actually, it looks like you can use some parts of it for free.
Also, if you have your windows firewall enabled none of these remote tools will work. Most of the ports they use are blocked by default. Someone here might know a way around the firewall, but it's probably more trouble than it's worth.