DJBOUTIT,
I assume since you're asking about software to screw shit up you ain't working offa turntables of any sort. In that case, any software that does decent timestreching will do what you need. Chopp it with whatever editor you wanna use. Tell us what platform you're using & maybe some software recommendations will roll in.
Maybe if you throw up a link to a screw mix or something, others can better help you once they understand what you're looking for. If I didn't know who dj screw was or what/how he did his shit, I certainly wouldn't just go looking around for it so that I could help you out. Not the kind of "research" I'd be willing to do (but that's just me).
good luck.
oh yeah... Do us all a favor and make sure that if you do get your screw on & drop some shit that you don't front like you fuck with the wheels proper if you're doing that shit on your computer.
DJs play records
(or CDs with a CD turntable)