Your monitors are probably logically (that is, in the computer's mind) set up like this:
+--------+ +---------+
| | | |
| | | |
| | | |
+--------+ | |
| |
| |
| |
| |
+---------+
Right-click on the desktop, and select "Screen Resolution". In the window that appears, you can drag the boxes around relative to each other. These boxes are your monitors. If you align them in there like they are on your desktop, it will be easier to move your mouse between the monitors.
For instance, my monitors are set up like this:
+--------+
| |
+--------+ | |
| | | |
| | +--------+
| |
+--------+