I bought sitemap generator years ago, and it sits unused for long periods. This is about the 3rd time I've had to reregister - and I cannot find the email with the code. Please help.

I think there might be some miscommunication, you should immediately contact their technical support team and get out of this problem. 

version 1.x = 1.x license file
version 2.x = 2.x license file
version 3.x = 3.x license file
version 4.x = 4.x license file
etc.

If you bought e.g. 2.x, you would be eligible to a free 3.x upgrade if 3.0 was released within an year of your purchase. (You would retrieve it online. See below.)

When 4.x was released, you would need to get a discounted/paid 4.x upgrade if you wanted to use 4.x.

So assuming you were to buy 4.x upgrade now, and 5.0 is released within an year, you would also be eligible to a free 5.x upgrade.

Both free and discounted/paid upgrades can be retrieved through:
http://www.microsystools.com/products/sitemap-generator/upgrade.php

I can highly recommend signing up to the newsletter to get reminded of free/paid upgrades:
http://www.microsystools.com/newsletter/

You can download old versions here:
http://www.microsystools.com/products/sitemap-generator/all-versions.php

Remember that for normal updates, like from 4.0.0 to 4.1.5, can be downloaded from the A1 Sitemap Generator front page without any need to retrieve new licenses since both version 4.0.0 and 4.1.5 belong to version 4.x series.