I agree with what SadieKate <EDIT - dial up takes a long time to load each page> and Corsair <EDIT> has suggested, see if the brakes are just skewed in which case they may just need a little wriggling/adjustment to get the pads away from the wheel...
It happens relatively easily - a little bump can do it, or if you have taken wheel off for travelling, or to change tyre or whatever... you need to make sure brakes are realigned.
If they are rubbing once or twice each cycle of the wheel (not constantly) it could be your wheel needs "true-ing", that is, your spokes need to be checked for loose ones and any loose ones need to be tightened - you should have a spoke key in your bike repair kit. Or perhaps you have broken a spoke without realising it - do a quick count and make sure they are all there.
Or, more expensively, you may have gone over some bump that buckled your wheel, in which case you'll need a new one...
I hope its just the alignment of the brake pads...