I don't think a computer ends up as 'clean' afterwards as with a fresh install; however, PC Decrapifier is much, much cheaper (free, in fact) and takes a few minutes to run rather than the hours to days it takes to install and setup an OS.
IMHO, driver disks included with a computer are nice as drink coasters, but worthless as disaster recovery assets. If your HDD dies, replace it and get the PC running again, then plug the model number into the manufacturer's website download the latest drivers for your hardware (if you have no other access to the 'net, if, on the other hand, you have another computer at your disposable...well, use it to get the drivers).
If you still want to go the fresh install route (which is, I cannot deny, a good idea) be aware that sites like newegg offer an OEM version of Windows, usually for about half the price as the retail version, the difference? No shiny box (seriously, that's it) - that said, a retailer may have restrictions on purchasing a copy, eg: must be purchased with a motherboard and CPU - however, you could try putting in your cart all by it's lonesome and see if you can make it out the door (stopping by the cashier on the way, of course)