Trouble with DPR cookies?

I'm using Windows 7 and IE 9. I've used Avast! antivirus and Spybot for several years. Until about a week ago, when I opened a post in DPR Forums, the print in the title would lose a little brightness, so I could clearly tell which posts I've read. I have somehow lost this handy method of tracking.

I'm not aware of anything I did that might have changed this. I tried resetting all the security settings to default in Internet Options, which had no effect. I don't know what else to try.
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Dan