Hello Pete,
Certainly there is an alternative, since there is no information on Xerox Phaser 3100 MFP compatibility with Windows 7 yet. The manufacturer is yet to release a driver for Xerox Phaser 3100 MFP to make it compatible with Windows 7. However, you may uninstall and reinstall the Xerox Phaser 3100 MFP drivers for Windows Vista in Windows Vista compatibility mode since you mentioned that it worked with Windows Vista.
Use this link to make older programs run :
Follow the steps in the link to run older programs in compatibility mode.
http://windows.microsoft.com/en-us/windows7/Make-older-programs-run-in-this-version-of-Windows
Azeez Nadeem - Microsoft Support