Tried to reduce proirity of rtvsscan.exe since t40 xp machine s running slowly. Access was denied. I am signed on as administrator.

Scan your computer with strong anti virus and anti malware tools because your computer setting changed or hijacked by virus or spyware and in such a condition It doesn't matter you are log on by admin or limited user, go for complete scanning your PC and you have one more option to work on your computer, make a new user from control panel> user account then create the user but it might be access denied.

That is incorrect. Rtvscan.exe runs as a system process which even the administrator does not have access to directly. Reducing the priority of rtvsscan.exe will not speed up your laptop, it will only cause it to run slow for a longer period as the virus scan will take longer. Try scheduling a virus scan for later at night when you are not using the machine or add more ram.