Cleric: Are you French/Belgian/Dutch?? There is a bug in MAMEWAH which means it doesn't like the decimal seperator from these countries (& possibly others). Go into Control Panel > Regional & Language Options and check your decimal symbol...if it is set to ',' then either a) change it to '.' or b) edit your MAMEWAH.CFG file and alter the Font Size setting to an integer (eg 10).
If this is not the case then let me know & I'll try and find the problem.
Neo Geo roms should work fine as long as you have the neogeo.zip in your ROMs folder. Just make sure that you have the latest version of neogeo.zip, it has changed a number of times (I suggest re-download it just in case).
Hope this helps...