you can perform various registry tweaks to change this information. Although if your not comfortable in the registry, then its not the best way to go. (incidentally, the tree is HKEY_LocalMachine > Software > Microsoft)
Download a copy of TweakUI, or TweakXP, and you can do it from within the software.
Whilst theres no need to reinstall Windows, it might be a good idea to do so if you've purchased a second hand laptop anyway (although quite frankly why anyone would want to buy a second hand piece of IT hardware is beyond me)
When buying second hand PCs' or laptops or whatever, it is usually a good idea to reformat it anyway, you dont wanna be running all the crap the person before you had installed (unless you want to ofc ).