just try it
if it's not legit, it'll tell you so.
nothing bad will happen.
nobody will knock on your door.
and you can still get automatic updates.
you just wouldn't have access to the update site itself
You won't be able to download items on the Microsoft web site that require Genuine Advantage. And newer items, such as IE7, that check during the install process.
Now to get a fully liscenced legal copy of windows call Microsoft and tell them who sold you the PC with an invalid or previously liscenced copy of Windows on it.
Microsaft will provide a legit copy for you and go after the reseller.
Any PC sold "with windows installed" is required to have a valid liscence. The reseller is guilty of fraud.