go to your Library.
That's the one that is inside your username folder, NOT the one at the root of the hard drive.
Go to Preferences and find the mouse preferences.
In my version of 10.6.x there are two of them. One for the Magic Mouse and the other for the original, wired, Mighty Mouse.
Drag whichever one you have, or both, onto the desktop.
If you have the Bluetooth Magic Mouse, you may need to do the Pairing thing again.
See if that makes any difference.
If no difference, try removing the Finder preferences, com.apple.finder.plist , using the same procedure as above.
At the reboot, a new Preferences plist will be created.