What version of WIndows are you running? This sounds like a pop up that's a result of malware infection on your PC. Sometimes your operating system will alert you to specific issues that may negatively affect performance (such as low memory, low hard drive space etc) although I can't think of a chart that would present itself.
Go to
http://www.malwarebytes.org/ and choose to download the free version. Save it to your desktop, install the program, update it using the in-built update option and then run a scan and see what it finds. It may find things and give you the option to remove them, do this and see if you still get the warning popping up. Chances are you won't.