You've not stated which version of Windows you're using, so the instructions might vary slightly. Basically, you need to access the Control Panel and then double-click on the Mouse icon.
In Windows ME (and, if I recall correctly in Windows 98), you simply go to Start > Settings > Control Panel to access it.
Windows XP (I believe) also allows you to access the Control Panel from the Start menu. If it opens with 'Pick a category' displayed, click 'Switch to Classic View' (which is easier to use).
As far as I know, Windows Vista also enables you to get to the Control Panel from the Start menu. (Microsoft has a nasty habit of making simple jobs harder every time it 'upgrades' Windows!).
Once you've found and double-clicked the Mouse icon, click on the various tabs and make your required adjustments. Then click 'Apply' and 'OK'.
Chris