I just renewed Spamfighter and wanted to purchase the SlowPc after running the free scan.
I had completed my Spamfighter purchase, but the invoice that generates after I click on Buy Now shows Spamfighter at $29.00 rather than the SlowPc purchase information.
It could be that you are on the SPAMfighter webpage and not the SLOW-PCfighter webpage.
Please take a look at this website, clicking "buy now" should allow you to purchase a key for SLOW-PCfighter:
http://www.spamfighter.com/SLOW-PCfighter/
Please let me know if you have any trouble
john