Originally Posted by Aimee G

That could be several things. If you're only getting it on one site, it's unlikely that there's adware on your 'puter. If it was a blog & that blogger paid for hosting & they didn't pay their bill, the hosting company can & will put an ad in that space until the blogger ponies up the dough to get their URL back. Again, it could be a redirect virus. Or it could be that the website you tried to access belongs to a "hub", & every *so many* (I don't know the ratio) times you access a website on that hub, instead of the website, you get an ad.

If you are having a problem on more than one site it could be that your security software is letting somethings slip through. Not every program is great at catching every type of Malware. If it continues, try layering your protection. CNet has some great programs you can download for free, & they're safe as long as you actually download them from CNet (not the mirror sites) Just make sure if you do layer that the programs "play nice" together :)