Your Steam browser can be hijacked to show ads and even redirect the store page.  Once you have removed the infection from the computer, the Steam browser may remain hijacked.  This is because of the web cache that Steam keeps continues to load after the system is clean.

To be sure you have removed the infection, you can check out my malware removal guide here.

How did my Steam get hijacked?

Steam has a version of the Chrome web browser built-in.  You have somehow contracted a malware infection on your computer that redirects web requests.  This type of infection will affect any web browser on your system including IE, Chrome, Firefox, and other custom browsers.

Why does it linger after removing the infection?

Your Steam browser uses a cache to speed up browsing, but it has cached the bad pages.  So any time you load up the Steam Store it will load the bad pages from the cache instead.  We need to clear that cache.

How do I clear this bad cache?

There are two locations in which Steam keeps its web cache.  You want to delete the whole cache folder and Steam will recreate a fresh one the next time it starts up.  Make sure that Steam is closed before deleting anything.  Find it in the tray near your clock, right click -> Exit.

Deleted the following folders:

steam/appcache/httpcache (This may be located in your Program Files folder)

%LocalAppData%/Steam/htmlcache

Fire up your Steam again and see how it went.