No, GC doesn't have text ads embedded in the forums.
Sounds like you have an adware/spyware program on your computer doing this or maybe even a virus. If it's adware, it might be a program called "Top Text" or possibly "Surf+"
Ad-Aware &
Spybot have already been mentioned above. I generally prefer Spybot over Ad-Aware, but both are very good programs. Doing your spyware/adware & virus scans in safe mode would be best.