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PS, now my normal GTA game wont even run..it freezes on startup..even when all i wanted to do was cruise around

Unfortunatly, i am addicted to GTA III like most of you, i hope....heres my deal. I'm a little slow when it comes to computers, i love gaming but i cannot get any of this MTA stuff to work for me. I D/Led the newest version of the MTA, but dont know how to login...Also does this MTA run off my already existing version of GTA 3?? ....Thanks for the help...Step by step would help...thanks lol