If you have Win 8 pro you are entitled to "downgrade" for free.. but only to Win 7 (IIRC).. if you have Win 8 normal.. you don't have the option anyway.
So you will need a XP licence.. which you will need to get hold of. And remember - MS will stop support in Apr 2014 for XP.. so no security patches after that date.
And if you have recent hardware.. XP may not support some features you want.. e.g. is you have a SSD you may want TRIM..
So unless you have a VERY good reason to go XP.. you should not go there..
i really don't recommend reverting to Window XP. It's a very outdated OS, most of times there are no drivers available, lacks several new features and has a lot of safety issues compared to Win 7/8 and can't play some recent games which run only on DirectX 10 or higher. You'll have trouble even when installing it fresh, because you have to slipstream specific AHCI drivers in the install disc otherwise you'll encounter a BSOD.
My recommendation is: if you don't like Win 8 use Win7. It's still an updated OS, can run all current software and it's almost the same thing as Win 8, except for lack of Metro UI (which you won't miss a thing) , presence of Start orb and some other features which were removed in Win 8.