The problem is nothing to do with mugen; rather it is a common error with XP. What happens is the system deletes the autoexec.nt file, required for all 16-bit apps. so it needs to be restored. 1. Do a search for autoexec.nt on your C drive, you should find it under "Repair" folder I think its called. 2. Copy this file 3. Paste it into C:\Windows\System32 4. Right-click on the new file, properties, make it read-only (stops xp deleting it again) 5. Now your mugen should work.