OK...this is gonna be kinda hard to explain, I've been playing guitar for nearly 6 years now and I've never had this happen so bear with me. So here goes:I have a steel string acoustic and the tuning peg on my guitar is somehow broken, because whenever I try to tune the G string - the peg keeps jumping and won't let me tune past a low E..quite annoying really...

So what I need to know is a) what is happening because I've never seen it before and, b) how do I fix it without going out and buying a whole new machine head.

Also...if it helps, I can see that it's only the top part which the string winds round that isn't turning...the actual machine head itself turns fine

Try taking out that G string and then look at the machine head, then replace the G string.

I had a similar problem, admitedly on a classical guitar. The problem was that the worm gear that drove the capstans had worn out so that when the string reached full tension it made the driven gear jump over the worm gear. There was no solution but to buy and fit a new machine head. The cost was small compared to the cost of the instrument and I was able to fit it myself. So I do not think you ought to be scared of changing the machine head.