TeamViewer +1
A friend of mine just moved his 42 remote WinXP boxes over to
TeamViewer. A friend of his is moving 38 WinXP boxes to TeamViewer.
I've only got 3 remote WinXp systems to worry about....

If you want to stay with Open Source, then some flavor of
OpenVNC/UltraVNC with VNC-Repeater/VNCrepeater might fit the bill. The
trick is to come up with a VNC server that the remotes all connect to
so you can bypass problems with opening ports in the remote firewall.

My friend's WinXP boxes are all behind home firewalls they don't
control, so having a central server they can connect to is the most
important feature of LogMeIn and TeamViewer. I tried talking to him
about other VNC & VPN options but there was no learning time so he
chose to go ahead with TeamViewer.

Doug.