#dpkg-reconfigure xserver-xfree86
on the first mouse screen choose:
PS/2
on the second screen choose:
IMPS/2
on the third screen say YES to Emulate 3-botton mouse
say YES to a new Config-4 file
finish the configuration, reboot.
For more complete information on how to configure X in Debian:
The Very Verbose Debian Installation Walkthrough
Sections 9 and 10
http://osnews.com/story.php?news_id=2016
Also take a look at the sticky at the top of the forum scree:
Debian Configureation Post-Install