EXP/Agent.b
This potentially malicious JavaScript attempts to bypass certain popup blockers (for example: Norton Internet Security popup blocker, Internet Explorer's popup blocker, Google Toolbar etc) in order to show popups. The script sets a cookie called PayPopUpAds which is used for tracking visited websites.
Don't sweat it, it's a "low" damage risk trojan.
Its common name is: Application.JS.ForcePopup.A
But is also known as: Trojan-Clicker.HTML.Agent.a, EXP/Agent.B, and
Trojan-Clicker.HTML.Agent.a#22.
To clean it from your computer, delete the cookie.
Run your anti-spy ware software of choice, install any new updates, then follow the prompts.