my computer is very slowly dying. It's over 5 years old, it's dying.

I want to format the hard disk and start again, but I don't have a windows key. (I have an xp disk, but the crappy bit of cellophane the key was once Stuck to has long since vanished). Is formatting and then installing linux instead(A) good idea (B) bad idea ? I have a novice/limited understanding of ubuntu.