You may be familiar with Windows Marketplace, which is one way you can sell downloads of your software online. As you would expect, you get paid when someone buys your software through the site. If you haven't added your software title to Windows Marketplace yet, see the Windows Marketplace Partner Page to learn how.
A little known fact is that you can actually get paid twice. By becoming a Windows Marketplace Affiliate, you also get paid a commission (up to 7.5%) when customers you send to Windows Marketplace purchase digital downloads of your software, and other titles, such as Windows and Office.
Even if you don't sell any software titles of your own, you can still become an affiliate. Then you can earn commissions by promoting downloads of other people's software through Windows Marketplace on your site.