Are computernames still in use.

I have a user base of 500 users approx but assigned computernames are up to 800. I reckon there are free PC names available to use since a lot of people have upgraded PCs, gotten laptops ect. Is there any way of finding out if these PCs are still active? Tried pinging computer names but there are too many to do individually and I would like to automate the process and record the information over a 4 week basis, there is a Wins and DHCP server onsite.
Any ideas gratefully received.