back oriface is a fantastic computer monitoring software... it hides completely transparent to the user... its actually a "controled trojan"
i use at work a software called big brother... all we use it for is to intermitently ping a device to see if its there and functioning. this is important to know because we have many customers. and because of the scale of the wireless network (that being many towns wide) its easy to lose someone and not know it till theres an angry customer on the phone. for the most part... its just a simple unix script and a lil tiney prog... that "can" interface with a fancey interface that is web page viewable... and configurable if you needed... and also emails failures to your email with the issue...
(there are many other things besides ping... but for ethical reasons...)
altris vision is one many schools use... i have it and an activation disk... but i cannot give it away...
there are also key stroke loggers that log and forward keystrokes unknowingly to a database, remote log, or email.
most of these are foil'able by simply disconnectig from the network... except the BB prog that will report when you are no longer present.
all of these will load your network down. the big brother and keystroke logger the least. but that depends if you configure them right... for instance... not ping every 10 seconds... and the keystroke logger to log for 15 minutes and then send a simple txt to an FTP server or something. this keeps packet size small, insignificant, and sparatic.
the flashier your monitoring prog... the more $$
apple has a prog called Anet (apple net) that will allow you to monitor whats on the screen... what is on the speakers... and what the microphone detects. it will also allow you to take over the computer... lock it down...talk on your mike over their speakers... or display another remote macs screen on that mac as if it were that remote mac. (excellent for schools) it also allows remote administration and configuration... need to clone 100 macs an get them on the network... this will do that too... clones hdd to look like the original... but reconfigures all the macs to have a given network set up if DHCP wasnt desired.
this is a pricey software...
but back oriface (is free) and a pirated copy of ghost will do the same thing for the PC side... its a lot harder to use... but will do what you need...
hope this helped
kyle