Help think computer has virus but Norton Anti-Virus not detecting any threat

Hi

 

I just bought Norton antivirus 2012 yesterday and installed it. I ran a full system scan and it removed/quarantined 6-8 files that were adware/ malware/ Trojan files. But the reason I got Norton's antivirus 2012 for is still happening, certain programs are unable to open or operate. These programs include: iTunes, windows defender, sony vegas 10, windows photo gallery and others. And all programs crash when I try to open or run any of them, it says: (Whatever program I to use) has stopped working - A problem caused the program to stop working correctly. This problem just started happening yesterday haven't had any problems before. I thought Norton would fix this but have done another full system scan and Norton doesn't seem to find any virus. So now I'm here and I don't know what to do.

 

I'm running Windows Vista Home Premium on 32 bits

 

Any help greatly appreciated

Thanks