If you want to reset all of the keys, delete c:\yourmamedirectory\cfg\default.cfg
Or open that file with notepad and see which key you set for UI_CONFIGURE (Right Shift + F4 in the example below,) run MAME, use that key to open the menu up, and reset it (clear it twice using whatever key you have set for UI_CANCEL)
You could also delete just the port entry for UI_CONFIGURE from that file
<port type="UI_CONFIGURE">
<newseq type="standard">
KEYCODE_RSHIFT KEYCODE_F4
</newseq>
</port>
Edit: Misunderstood "I'm a windows user" to mean "I'm a windows user" instead of "I'm usually a windows user". My fault for not catching which board I was responding in. The info is valid, but it doesn't answer the OP question.