Unable to Install Antivirus

Hello,
I have a long running virus in my system. i didn't use any antivirus. now it created problems.

Firstly I cannot start my Pc in safe mode. i am using XP.
and second, I am unable to install an anti-virus and many other programs. Even my Taskmanager is Blocked and it says "Task Manager has been disabled by the administrator".

Please Help!!!