If you try to install the Cisco VPN Client on a Windows XP that already has the Watchguard Mobile User VPN client you are in for a treat.
The Cisco VPN client installer will keep telling you must first uninstall the previous version or uninstall a component called IRE (I think that was it). Of course you can't install it because you haven't installed the client yet. That can be quite misleading.
Well, just uninstall Watchguard's MUVPN client, install Cisco's and then reinstall MUVPN. Now you have both installed, MUVPN works but Cisco's client will complain: "IPC socket allocation failed with error -4h (...)".
To be able to use it you must kill MUVPN processes. Just right-click the taskbar, fire up the task manager and kill the following processes (you must be administrator): "SafeCfg.exe", "IreIKE.exe" and "IPSecMon.exe".
If you want to use MUVPN after that you will need to reboot or figure-out a way to re-start those processes. I didn't have patience to find out how; it might be as simple as running the executables. Good luck.