If you are switching between the USB Mouse and the USB/PS/2 adapter, you need to make sure that everything is configured right.
For PS/2 Connection:
In your /etc/X11/XF86Config-4 file, make sure that in your INPUT section, the mouse device points to /dev/psaux and that the Protocol is "PS/2".
For USB Connection:
You need to make sure that your USB subsystem is up and running, most times a distribution like Mandrake and Redhat will do this automatically. The device to access a USB mouse is most likely "/dev/input/mouse", to test to see if the system is reading the mouse, type:
QUOTE
cat /dev/input/mouse
...and move around your mouse, you should see a bunch of garbage characters fly accross the screen, this shows that your mouse is in working order. In that case, you can edit the file mentioned above in the same section and change the device to /dev/input/mouse .