I have ran scans on fully updated Spybot Search & Destroy, AVG, and Avast, and non of them have come up with anything.

Every time I run the Steam application, ads keep popping up on the bottom left of the screen. This is the built in Steam application, NOT my internet browser ( which does not have this problem ).

The Steam community is useless on solving this issue, and Google turns up nothing useful on the subject. As far as I can tell, there are no processes attached to this bug. Any idea on what it is, and how to get rid of it?

[mod edit: removing NSFW image host --rip-off]

No idea how to solve it but that last add made me lol

But it's not a joke!

Warning to visitors: Screenshot 3 is NSFW (not safe for work)

Have you tried reinstalling Steam?

Almost certainly malware. Me, I would try using something like Process Explorer to investigate further, having exhausted Google.

Weird.

Not that I can be of much help, but just out of curiosity:

• Do you get this when running Steam, or when running the Steam overlay (Shift+Tab from within a game), or with both?
  
• Do you get the same results when visiting http://store.steampowered.com/ ?
  
• Have you tried erasing the Steam html cache? (I'm not sure where it would be located).
  
• Have you tried uninstalling/reinstalling Steam? (but be careful not to erase your saved game files and downloaded games)
  

Warning to visitors: Screenshot 3 is NSFW (not safe for work)
[/quote]
All the images seem to be accompanied by NSFW ads, which "postimage" is apparently known for. I've removed them for the moment.

Steam makes use of Internet Explorer internally, so it looks like your IE / windows browser components are infected with a virus.

Time to reformat...

NOT my internet browser ( which does not have this problem ).

Is your browser IE? If not, check if IE also has this problem.

My default browser is Firefox. As far as active processes, nothing looks out of the ordinary.

When I have more time, I'll have to track each one down.

Here is the screen shots again - this time directly linked to skip the inappropriate ads PostImage puts on them.

Here is the Java Script that displays the ads. Still no clue what injects it into the pages.

This is really weird.
Searching through google I did not find anything like this.

Even with you saying that the Steam Community didn't solve your problem, it is worthy to create a support ticket and warn them about the problem.
I think it is more related to IE as Hodgman posted. Try doing more tests searching the web using IE (entering the steam store online, etc.) and see if the ads pop-up.