Your computer remembers almost everything you do, and so does Google.
Deleting/clearing these old files is the most common issue with viewing your ads.
Please click on the link below that matches the internet browser you are using for step-by-step instructions on how to do this.

Internet Explorer: http://browsers.about.com/od/internetexplorertutorials/ss/ie8privatedata.htm

Google Chrome: http://www.google.com/support/chrome/bin/answer.py?answer=95537

Mozilla FireFox: http://www.eraseallhistory.com/erase-history-free/erase-history-Firefox.html

Safari (for Mac): http://browsers.about.com/od/safar1/ss/safarihistory.htm