Steam has a two-tiered system for user accounts.
An account that is just registered, but doesn't have a paid product on it is considered a Limited User Account and has some restrictions placed on it - mostly community-related. This is for security reasons.
The previous "one product registered" badge has been replaced by the Game Collector Badge in the update on June 18th '13:
Game Collector Badge: There's a new badge which will grant you XP based on how many games you have in your Steam library. This badge replaces the "100 XP for having at least one product registered" message at the bottom of the badges page.
The following information is now outdated, but will be kept for reference as the question itself is no longer relevant.
This is not a badge per se, it is just a way for Steam to list it under the XP granting activities. It doesn't have an image and it doesn't have any special text to it - it's just that:
For having at least one product registered
You, therefore, receive 100XP just for being a real person in the eyes of Steam. Congratulations!