You've been nailed by adware; what you describe (the keyword underliner) is just one of many sinister types floating around.
Get yourself AdAware (
www.lavasoft.de) and SpyBot Search and Destroy. AND BE SURE TO KEEP THEM UPDATED!!!!
Don't uninstall anti-spyware programs once you think the problem's been solved. These scumsucking marketers look for ways to fool or even disable anti-spyware programs to bring their polluting message across.
Also be sure to go to Windows Update and download any and all software fixes and service packs... it's amazing how few know about this, which would control or eliminate any potential security holes in your system.
Spyware - software that tracks your computing habits and surreptitiously transmits it to another user or company.
Malware - software that's poorly written and sucks up too much memory for what it claims to do. Also likes to take over the registry by making itself the preferred program, whether you like it or not.
Adware - software that is nothing but a channel to bombard you with ads for useless shit. Some are as insidious as keyword underliners, others just throw a shitload of popups at you.
Browser hijackers - some websites (especially porn sites) will attempt to change your default home page or "mousetrap" you (short of giving the three-finger salute, you can't click back out of the site).
Hope this helps.