Suspicious system popup window "Restricted" asking for username and password (http://client-projects.com)
I'm getting a suspicious system pop up window on random websites (weather.com, walmart.com, etc. no regular pattern noticed) that says "A username and password are being requested by https://client-projects.com (which is hyperlinked, but I have not clicked on it and don't plan to). The site says: "Restricted" and then a space to enter a username and password beneath." I've run a virus scan with my AVG, and two malware scans with Malaware and Spybot, and nothing major has come up and the popup is still occurring.
When I try to search (Google and Bing) to see what http://client-projects.com is, the search says the website is blocking the search engine's description, which is a big red flag for me. I've disabled Shockwave in my Firefox, and the pop up appears to have stopped for now, so am updating Shockwave, but I'm still concerned there might be something nasty on my computer. How can I check further to try to track it down and/or does anyone else have any info on this popup or website?
AVG Virus scan
Spybot Search and Destroy
(VS and Malware scans are ran regularly and kept updated)
Thanks for any help/advice.