Problems with my rear derailleur
 

On the rear derailleur on my mountain bike (I think it's a Shimano Deore 7-speed, I can check if needed). The chain will rub against the outer cage plate down by the tension wheel if it's in on one of the outer gears. And if it's on the first gear, it the inner cage plate will hit against the spokes of the rear wheel. Does anyone know why this is happening, and how to fix it? I don't know if something's put on wrong, or something's bent, or what the problem is.

Also, the chain on the bike was rusted over, so I replaced it with the chain from another old bike that was lying around. I'm not sure if the chain matches, so what should I look for to make sure it's adaptable? As you might be able to tell, I'm looking for a cheap fix, and would rather not buy a new chain if avoidable.

Thanks for your help.

Sounds like it could be a bent derailleur hanger (if it has one), or frame (if der.mounts directly to frame), or the der.arm could be bent. Best thing to do is avoid guesswork and take it your LBS and have it checked out. While you're there, have them check to be sure the chain you put on is in good shape and is the correct length.