DJ ProMixer: the €1.99 mixing software
If you want to start DJing on your PC - but you're not keen on paying much money to do so - Digital Multi Soft's DJ ProMixer, which costs just €1.99, could be the software you've been looking for.
This offers support for a wide range of hardware controllers (include models from Pioneer, Numark, Hercules and Vestax) and can be optimised for use on slower computers (netbooks, for example). It promises tight syncing of tracks (down to one thousandth of a beat, apparently), a multi-function wheel that offers access to a range of features, a mixer, sampling and effects.
The interface looks like it adheres to a fairly standard twin-deck blueprint so you should be able to get up to speed with the software pretty quickly.
Check out the DJ ProMixer website for more or have a look at these tutorial videos.