Here, I'll show you the site (normally you can't see this page without making an account):

I picked a random car to show the whole page. Usually, a customer would call in, and if they ask about a part, the receptionist can click the orange 'quick labor lookup' button on the left, and a window will open where you can enter more information. That button, along with the 'find it' buttons under each item in the picture above, no longer open a window. I looked at the browser options, and pop-ups aren't blocked, so I decided it was the antivirus. I uninstalled AVG Free, and lo and behold, the buttons worked again!
Unfortunately, they stopped working again after I installed Avast! Antivirus. I tried disabling Avast, but the buttons are still broken.
As far as I've looked, no other buttons or websites are broken, and the problem just started in the last week. As far as this guy knows, nothing has changed on the computer besides the contents of his QuickBooks files, but something must have gone wrong. Does Avast have a pop-up blocker or some kind of javascript protection that I can disable?