Please just be careful when you Google for help. Some of those sites are actually site-hackers: take over your machine and ask for money to remove your problem and give it back to you.
If you don't get rid of it it can really mess your computer up. Good advice from all re: antivirus software, but this trojan got past mine and finally showed up on a later scan. Keep your software definitions updated and scan if you even suspect your protection has been breached.
I love this Board and it is a shame this is happening. For a while I will be shutting down my computer at the first roll-over or pop-up and run a scan. And I will try to identify the ad(s) that is causing this and report it to Administration per their instructions.