You simply need to use Telephone activation -
this is to confirm that the conditions of the license are still being met.
Assuming your activation goes OK, then you will be able to activate over the internet again if necessary.
However, if you frequently reinstall on the same hardware, you would be far better to create a default install and then make an image backup of that, and restore it, as that will then contain all the proper updates etc., and your default programs - tehreby cutting down the time and work involved in gettting the system back up and running satisfactorily.
telephone activation (operator calls)
Click on the Start button
in the Search box, type
and hit the Enter key
follow the instructions - but when asked which service you require by the telephone ansafone, do NOT reply - this should force an operator to respond, who can deal with you