It's either a parasite or you installed a feature from Officexp. Here's the treatment for pests.
These Parasite removal programs can also SCAN. CWSHREDDER pretty much just removes such, but the others can just scan then ask if you want to remove such.
However, my policy needle on the pest-meter is pegged at zero tolerance. I eject all that these five tools find.
http://reviews.cnet.com/5208-6132-0.html?forumID=32&threadID=1313&start=0 notes the five scanning tools and removal tools that are fairly safe. If one does encounter an issue, its always been damage done by the MALWARE. The most common issue being Layered Service Provider replacement which you can use LSPFIX (Hint: use http://www.google.com to find).
The thread has follow on discussions about new links if you don't want to use Google to find an item.
PPS. All the tools are free.
Suddenly certain keywords are being underlined in yellow, with links to websites. I frequent several cooking forums, and certain keywords on those sites are now being highlighted, whether it's part of my own post or someone else's. When I click them on, they link me to a site that sells something related to that keyword. Example, the word "dish." When I see it on any forum, it's highlighted, and links me to a site selling satellite dishes. Another example is the word "diabetic". It links me to a site selling something for diabetics. (I must have anwered someone's request for a diabetic recipe at some point). You can see the yellow underlined links on this exact post right here. If you click on the keywords "dish" or "diabetic," they will link you to the sites. What's going on? How do I get rid of this????

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic