You only need to pay once as you already have a DimadSoft account. You can redownload as many times as you want to the same computer. If you reinstall your PC, the Computer ID may be changed, then you need to redownload again with the new Computer ID.

In fact with the purchase, you can download up to 5 machines, but there is a 48-hr waiting period only for the first time you download for each subsequent machine. Also, according to the license agreement, you can only use the program in one of these machines at any given time.

Here's a little tool to get the Computer ID for the PC: http://www.dimadsoft.com/dl_comp_id.php?ver=2

Another download restriction discussion: http://club.cdfreaks.com/showthread.php?t=150012