aight, you have 2 options that i know of if you wanna do this yourself.
both are free.
first, you can get turntablist pro vst plugin.
http://www.kvr-vst.com/get/163.html
this one works with your midi keyboard if you have it, you can scratch with your mod wheel.
second, get this little stand alone proggy.
http://www.hitsquad.com/smm/programs/Scratch/
there is a difference in these two packages in how they scratch, one is more suited for certain things than the other and vice versa. Just get to know 'm both. Just like a real DJ with a turntable, you have to *practice* to make it work. Just mess around with 'm for a while, you will get the hang of it. with the second proggy you can record your scratches into a wav, then edit in a sample editor later (e.g. slice up).
Peace
p.s. dont be prejudiced because this is software, i guarantee you you will be impressed with the scratches you can make with these proggys, especially since you are not a turntablist (and even if you were, i dont think you would be unimpressed). Until i get my own tables some day, this is gonne be dope enough for me.