Adware has found its way onto your PC, which is designed to open a window displaying an ad. However the site which the window was meant to display no longer exists.
Run your anti-malware program (er, you have got one, haven't you?) to see what it finds and (more importantly) can remove.
The most popular programs are Ad-Aware Free
http://www.lavasoft.c...cts/ad_aware_free.php
Spybot Search and Destroy
http://www.safer-netw...n/spybotsd/index.html
and MalwareBytes Anti-Malware
http://download.cnet....-8022_4-10804572.html
Chris