Windows Security Center displays that my AVG is off and Windows Defender has problems?

I have Windows Vista Home Premium and I recently downloaded free software off the internet. However before downloading I checked the sites security certificate and had a rating of 80/100. So thinking I could trust it I downloaded. Just a few minutes later

I noticed that it was naff so uninstalled the program. About a few days later I realised that my web browser kept coming up in annoying pop-ups so I checked online and found that I had installed a worm. ( I think) and removed the adware from my computer. I

then done a full scan on my AVG 2012 and it came up with nothing. After lots and lots of full scans later there was still nothing. But now my Windows Security Center has notified me that my AVG is turned off and my Windows Defender is problematic. I think

I might have a virus but no matter how many scans I do it doesn't come up with anything. As with the Security Center, I try to turn it on but it won't let me. (I found this was common and as I know my AVG is on I selected the &quotI will monitor myself&quot button)

The defender now occasionally comes up with annoying messages but when I click on it, it says my computer is protected. Now I have found that my AVG won't scan anymore. How do I solve this or get rid of a possible virus????

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