Unable to uninstall the Microsoft security essentials antivirus

Hello,
I am currently using windows vista on my system. The antivirus I am using is Microsoft Security Essentials, but now, I want to switch over to Panda antivirus. I tried to uninstall the Microsoft security essentials antivirus, but the system doesn't let it go. An error appears on the screen, which shows only the code, and that is:
'code 8007066F'.
How can I understand what this number means when it is not described properly? Not even a single word is written for describing this code.
Can anyone help me resolve this problem? How can I uninstall this antivirus without resulting in any errors?
Any kind of help will be highly appreciated.
Waiting for your help.
Regards.
