How do provide proof of purchasing VB?

[Gutspiller]

I don't like it when I get emails from Vbulletin.com accussing me of not buying their software. I received the following message. Which I would like to mention they don't mention HOW to prove that I have a legit copy of VB.

We are the attorneys and Internet Copyright Enforcement agents for Jelsoft Enterprises, LTD, the publishers of vBulletin software.

On February 18th, 2003 we contacted you regarding the use of vBulletin software on your web site and the fact that it does not appear as though you have a valid site license for use of the software. We have not received a response.

Please take note that if the licensing issue is not resolved by March 6th, by one of the following actions (1) Your providing proof of a valid site-license to use vBulletin; (2) lease or purchase of a valid vBulletin site-license, or (3) removal of vBulletin from your site, we will be filing a formal Notice of Copyright Infringment with your web host and requesting that access to your site be disabled.

I would highly recommend that you do not ignore this notice, as it is the last one you will be receiving from us.

I don't have time to investigate how to do prove to them I bought a copy and I don't like the fact that they threaten to take down my site if I don't provide proof of purchase (which they don't tell me how to provide)

Anybody help me out so I can get the VB nazis off my back?

And VB team, please fix your software than 'scans' websites for no proof of purchase. When I registered I was never told I had to signup or prove that I bought VB.