Windows Security Center should NOT be prompting you to buy any software. What you described, is typical behavior of a rogue anti-spyware program. The warnings are FAKE. The below removal guide may not be the exact rogue you have installed, but the instructions for its removal should help. All the tools are FREE.
Scroll down to where you see "Automated Removal Instructions for Security Center using Malwarebytes' Anti-Malware:" and follow ALL the instructions. Do NOT click on any ads.
Remove Security Center (Uninstall Guide)
If you run into any problems along the way, please post back and ask.
Best of luck..
Here is my situation: when I got the new Mozilla Firefox 4, I was notified of having viruses, malware,and etc (18 of them) on my computer by the Windows Security Center. When I run a full computer scan with the free AVG Anti-Virus software, it did not detect anything. The Security Center keeps prompting me to buy the software to help me remove the threats. This is an expensive option for me, do I have any other options?
Thanks in advance for your time and help.