They mean the same thing — cheated by someone, or put in a bad/disadvantaged position by someone.

This is my take of the word "screw" and it's usage; To screw somebody over: The seller is trying to screw me over (The seller is trying to cheat me). To be screwed: I'm permanently screwed by her selfish decision. (I'm permanently in trouble because of her selfish decision) To screw up: I'm sorry, I screwed up. (I'm sorry, I've made a mistake) To screw somebody: He screwed me (He had sex with me) Hope that clarifies things up.

I think the addition of "over" suggests "completely, from the beginning to the end". John's defintition is very clear. If you simply use "screw", it could have several possible meanings (including the obvious sexual meaning), so it might be difficult to understand the figurative meaning without context.