If you are running Vista and your USB mouse stops working, then this could be to do with Vista update KB938371.  This update, apparently, can disabling HID-compliant input devices, therefore here is a solution or work around.  If you are using a Laptop, leave your non-working USB mouse plugged in and use the laptop's touch pad to follow the below instructions.  If you have a Desktop, then likewise, but use a PS/2 mouse or navigate with the keyboard.

1. Click on the Start menu
2. Right Click on Computer and then Left Click on Properties
3. From the list on the left, Left Click on Device Manager
4. Now look for Unknown Device or your mouse device with the Exclamation mark by it
5. Right Click on the Device and Left Click on Update Driver
6. Left Click on "Browse my computer for driver software"
7. Enter "c:\windows\winsxs" for the driver location and Left Click on the Next button
8. If you get a popup saying "this is an unsigned driver..." then just allow Windows to install the driver

Once the Driver has installed, you should be able to use the USB mouse.  Once operational, simply pull out the USB mouse and then reinsert it into the same USB port.  If new device is detected, follow the recommended route for installation of the driver and this should resolve the issue.

If you are using a Laptop, you may want to try your device in each USB port.  Be prepared that you may have to follow the above instructions for every port.