Spyware and multiple users on a shared PC

When running spyware and adware tools (and AV tools also, I guess), is it ever necessary to run any or all of these applications for each user account on a shared PC, OR will the applications detect and remove malicious software when executed by an administrator?

On a tangent...I want to thank those of you who make this site available? I have found it to be a highly usefull knowledgebase and great source of various applications. I recommend this site often to people in need of PC assistance.

Thanks,
Jason