Microsoft Security Essential wont let my dowload
This problem is not related to Microsoft Security Essentials , it is browser problem or there is problem with Windows. I suggest that you update Windows and also install service pack(if any).
This message is also indicate that you might infected with a virus call : SirCam virus.
More detail please visit :
To troubleshoot, if it is Virus then you should update Microsoft Security Essentials and also go to help and check for upgrade ,then run full system scan and it should be detect and if that is the case remove it.