My client has a Mac at home and wants to remote control (run apps) on a Windows XP PC at the office. What is the best software to enable this? Any Mac <-> Windows weirdness to be aware of?
The best things will be to use the RDP client for mac.
RDP client for the Mac is of course the best option providing you are already have a VPN/Port forward or you are able to set it up.
Personally I would go for Logmein (free) they have an excellent Mac Firefox plugin. The advantage is you don't have to worry about the network side of things. It's also SSL secured if you worry about those sorts of things!
The best solution I've found is CoRD. It's free, open source, and it supports multiple saved sessions, shared clipboard, and a bunch of other features.
I've been using it to connect to terminal services on the server farm at work, and it works like a charm.
In fact, it's way better than the official Microsoft RDP client for Mac.
Don't forget about VNC - an oldie but a goodie.