Has anyone used win2vnc (or simular) to control a Mac and 2 XP computers from a single keyboard and mouse? Is that possible?

http://synergy2.sourceforge.net/

Yep. It's ace. Go for it.

Ah yes, I saw that too thanks Matt. Was struggling a bit (a lot) to set it up. Is it worth persisting with? Compatialble with mac and xp?

I'll be honest, I've only used it between Linux and XP but it did work like a charm - I can't see it being much different on a Mac and a lot of the guys at work here use it between all three.

Basically you need the server on one machine and the client on all the others. Then configure the positions of the client screens on the server version (I think this is allocated via IP - i.e. the screen for machine 1 is to the right of the server screen when it's connected and the screen for .0.6 is above, or whatever) and you should be ready to go.

Once it's set up it works flawlessly, or at least it did for me though having to specify the relationships both ways threw me for a bit. If you tell the server that screen 2 is to the left of screen 1 then all well and good, but if you haven't told it that screen 1 is to the right of screen 2 then you won't be able to move back!

The documentation on the site looks pretty good (http://synergy2.sourceforge.net/running.html) but if you post back where you're stuck I'll see what I can do to help.

The VNC server built into OS X - only, I think, 10.4 and above - should be fine on a LAN. If you want a snappier response from the Mac - not least on 10.4 - then install (the free) Vine Server. (That also has the facility to connect as a specific user - and hence not be seen by the currently-active user if different - as well as the option to connect to the current desktop only.) RealVNC fails to connect to OS X over the Internet in my experience but TightVNC does the job. Note that if you're going over an untrusted network such as the Internet you should connect via an SSH tunnel - more about which I can tell you if need be.