when im on my username on xp it won't let me play gta3, but then when i get on my bros username it lets me play. Both of our names are set to admin or whatever. so i don't know why suddenly it started doing this. i installed it originally on my name.
I bet you installed the multiplayer patch
Try this, goto your gta user files folder (C:\Documents and Settings\User Name\My Documents\GTA3 User Files) and delete all files except for your saved games GTA3sf(1-8).b