iv just bought a USB guitar link to record my guitar to the computer, im wondering the best way i would be able to play say an original track and play my guitar over it and record so that you still have the vocals and other insturments but with my guitar

Not a clue... Your best bet is a backing track, although I don't know if they have vocals as well...
Quote by Kensai

Awesome guy right here
If you have the software to hear your guitar through the PC, then Audacity is a free software that can record your original or backing and then let you record more tracks for your own sounds. There is a latency that depends on your computer and sound chip but its quite easy to synchronize after the recording. Then you can add post production effects and do a nice mix to balance it all up.
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yea i have the hardware for my pc to connec tto the guitar, i also downloaded audacity but not quite sure on how to use it the way i want to