If you are using a separate router, you could try cleaning your cache. Turn off your modem and unplug the router from it's power source. Wait awhile (give it a good 10 minutes), turn the modem back on, once it's connected plug the router back in.
Or, you could have spyware!
Download, install and update and scan with the following free:
Spybot Search & Destroy from http://www.safer-networking.org
Ad-Aware 2008 Free from http://www.lavasoft.com
SUPERAntiSpyware Free Edition from http://www.superantispyware.com or http://www.download.com
Malwarebytes' Anti-Malware...
Valid downloads of Malwarebytes only come from these sites:
http://www.download.com
http://www.majorgeeks.com
http://projects.securitywonks.net
If all else fails I suggest you register with the Spybot Search & Destroy Forums and post your problem in the Malware Removal Forum... they are excellent at talking people through the process of cleaning their computers. http://forums.spybot.info/