It depends on what your two systems are. If one is linux and that's the one you want to hack, try finding some proof-of-concept code for exploting a linux box from a windows one (which aren't as common). However, if both of your systems are of the unix/linux/solaris/bsd/*nix variety, then you'll have a lot more success. Simply browsing packetstorm, bugtraq, cotse.com, and googling should provide you with enough tools to start overflowing buffers and causing DoS on your target box.

Goodluck with what you want to do.

Peace,

Raz0rSharp