"....... contained a virus and was deleted"

Hi guys, I need your help,

Firstly I cannot download HijackThis because whatever I download I will get a message ....... contained a virus and was deleted.

I suspect my computer is infected with Zeroaccess Rootkit after some googling. However I am unable to download any malware program advised by the website that I googled as my computer will auto delete them.

Even my Microsoft Security Essential is not working. "Windows cannot access the specified device, path or file, You may not have the appropriate permissions to access the item. When I tried to uninstall it, it says " You do not have sufficient access to uninstall Microsoft Security Essentials. Please contact your system administrator.

Please help me T.T