Tech Support Alert Web Pages like these that "pop-up" on your PC's desktop are nothing more than an invitation to get yourself scammed out of your identity.
If you call this "Toll Free" number you're going to get a guy speaking in heavily accented English who will ask you a bunch of standard IT Support questions: what operating system, how old is your computer, any new software installed, etc. Eventually he will explain that he will need to remote into your computer to see what is going on. If you let him do that you are toast. From there he may explain the cost and ask for your credit card or just get right down to seeing what's wrong remotely. Once on your computer he can install a key-stroke logger that will capture everything you type including usernames and passwords. While he's there he'll can also scan your computer for personal information including bank account information, social security numbers, and credit card information and copy out folders and files. The best case is that he'll whack your credit card for some crazy fee then simply uninstall the malware that his infected link installed. Worst case is that he steal your personal data and your identify will be his. Most folks will see this pop-up for the malware that it is, but I guarantee you it will work on enough people to make it worth while.
This alert was likely triggered by an embedded link within an advertisement on a (probably completely legitimate) website you viewed earlier in the day. When you see malarkey like the image below just stop, close your browser and contact your known and trusted support resource. If you are unable to close it you can use Task Manager to do so or simply restart your computer. Contact your IT Administrator or Help Desk to alert them to the issue and for assistance.
The best way to combat this sort of attack is with web filtering services that protect your computer from ever reaching sites like these. Anti-Virus isn't enough anymore. The bad guys have learned new ways to get to your personal information without having to get through anti-virus software. Now they just go around it.
The Federal Trade Commission has some useful information and a fun video on their site that provides additional information on the Tech Support Scam.