Skurk,..If it is the corporate version, the time started when the license was acquired and is embedded in the installation key. Basically, if the company that purchased the license lets you go or goes under, then at expiration time you will have to purchase a new copy of XP, or whatever they are shipping.
You can read the license agreement from MS to find all this out, by the way.
If it is the hacked copy, then you better not be using MS Upgrade site, as they already have a patch to render those hacked copies invalid and will be releasing it in the next service pack for XP.
Oh, forgot to add, if you have a BETA copy of XP, which did not require any registration, it will also be updated to require registration in the new service packs.
Those pesky demons at MS are not missing a trick when it comes to makin ya fork over yer money. Too bad they are not as attentive when it comes to security and quality of product.