If you used a bootleg PK previously and now purchased a legit copy of the OS
and want to activate it, yes, you can do that without doing a complete
re-install. HOWEVER the new copy has to be the same type of install CD as
the original one you used. OEM will only work with an OEM PK, Retail will
only work with a Retail PK, etc.
Go to Start, All Programs, Accessories, System Tools and pick Activate
Windows. If it hasn't been activated yet, it will allow you to plug in the
new PK here.