I got a private message on a forum which invited me to join a membership which allowed you to download other peoples' software for the price of the membership.

I don't know who to contact to shut the website down? Is that possible. I know that it's probably been replicated across the internet but it might be a start.

Hackbridge

I keep seeing sites offering this after following spam trails.

It seems to me that, quite apart from the known criminals,
any site which spams using fake 'from' addresses and fake
or fudged URLs is not to be trusted with anyone credit-card
details, or even their email address.

I recommend reporting such sites by posting spam here to
SIRT. You may also wish to forward it to spam@uce.gov

For Microsoft products, go to
http://www.microsoft.com/piracy/reporting/default.aspx

In the UK report scams/fakes etc. to:
trading standards at http://www.ripofftipoff.net/

hth.