I would like to recommend
http://www.kaillera.com/ this program allows you to play mame games online, its a blast.
I read that someone was having problems with metal slug series, I have found that for all neo geo games the neoragex emulator is the way to go. I regularly use it for the metal slug series and it works flawlessly. It also has the advantages that the romsets are much smaller and load times are much better.