Not too much you can do... disable file/print sharing. Install a software firewall and block all machines on your network. Especially the "admins".

Even then... he could sniff your traffic weather you are using a hub or a switch. Depends on how smart this "admin" is.

You could put him on a different switch than the rest of you... which it'd make it harder for him to sniff because it would be a different segment. Though, not impossible.

When possible, encyrpt your traffic... but IM services don't have this yet.

Even after all that, the "admin" could gain physical access to your machine and then install a monitoring system. Physical access == no security. Especially with a 9x box. All you have to do is press escape at logon screen and it'll bypass your passwords. Not to mention all the other ways they could access your harddrive. Or BIOS password which can be reset.

I think you might have to get out the baseball bat.