This is quite a tough thing to do Sam, it's not at all like you see in the movies.
First of all you need to identify their machine from all the others on the internet, so you need to find-out their IP address. It's most likely that they get assigned a new one each time they connect to the net.
If they keep the files that you want ot look at in a shared folder then this makes things easier, for one this is not illegal to view them as they have put them in the public domain, and you can just map the network drive.
Other than this you're getting into the realms of installing or getting them to install a program that will give you access to there computer. There are plenty of these types of progs floating around on the net, some are commerically available (eg. remote administrator) some are free, created hack teams (eg. back orifice). Either way this is illegal.
Virus sofware doesnt prevent hacks, but it can spot them once there on the computer. A firewall can make it more difficult to gain entry to a target computer.
btw, I'm assuming that the guys running Windows, a unix based OS is slightly different to gain entry too, and I have no idea with Mac OS |