I'm not sure you can buy a single copy of Autocad anymore. When they switched over to desktop products about 7 or 8 years ago they switched to a subscription service which includes upgrades every year.
They no longer issue a hardware key with the product, after you install you have to request one on-line to activate the program.
I think your ACAD 2000 may be the last year of the single use license but I could be wrong.
You can still buy single versions without the subscription. I have Autocad LT 2008 and Viz 2008 with no subscription. I went from Autocad 14 to Autocad LT 2008 and spent a whole day just turning crap off.