Security Shield My computer, upon sufring the net, had a window pop up that said my computer was infected with spyware and security shield would not let me reboot or allow mcaffee to run to find the source of the pro, Internet

In March of 2012 my computer, upon surfing the net, received a pop up window claiming it had become infected with malware. I attempted to try to turn on my own anti virus software to no avail. Security Shield kept beeping in the pop up and would not allow me to reboot or otherwise until I paid for their software. I have since downloading the software and paying $80 not been able to run the program anymore and I can get a hold of no one to tell me how to fix this and or uninstall said program.