Does it not even show up in the control panel?
If it does, then:
Start>Devices and Printers>right click on the mouse
select mouse settings
click the pointer options tab
adjust the pointer speed
This will only affect MAME if multimouse is not enabled in mame.ini
If multimouse is enabled, MAME will get raw input data from the individual mice before windows does anything with it.
If multimouse is not enabled, MAME gets the input from the "system mouse" downstream after these settings have been applied. (but there is only one system mouse, so you can't have multiple mice....at least not in windows 7 or older)