task scheduler in Vista: run script at startup on any computer and any user in network

Hi everybody!
I have a problem, which demands some creatvity, I think:
I have a big computing task, that a single computer needs about 5 weeks to perform. As this computing task needs to be repeated several times a year, I wrote a script in vbs, which enables splitting up the task for all computers in the network. This should cut down the computing time to 2 days or so.
By running the script "start.vbs", a computer joins the calculation, running "stop.vbs", it stops. Idle computers shall join the task automatically. "Idle" means in this context: computer is up and running, but nobody is logged on.
That's why I want the following events:

1) at computer start up: run "start.vbs"
2) at user login: run "stop.vbs"
3) at user logout: run "start.vbs"
4) at computer shut down: run "stop.vbs"

More details and some background information:
Most PC's in the professional administrated network run Windows Vista, some run XP. The scripts work on both. It's no problem to start the scripts manually (by double-click), while being logged in. Goal is to automize the participation of the computers.

It is absolutely necessary that a connection to the common network-drive is established (requiered for the common computing of the task). Also "start.vbs" and "stop.vbs" need to be accessable on every computer.

Solution: (?)
I gave the Windows task scheduler a try - not sucessfull of course ;-)

Problems / Errors:

a) Works only if the *.vbs - files are on the local C-drive. (I tried UNC-paths, but didn't work).

b) Troubles with the usergroups and computers: Any user can choose any computer in the office. The task-scheduler doesn't allow me to make accordng settings: It works only if the task is related to my own account.

c) I find the events "startup", "login" and "logout", but I can't find a "shutdown" - event.

Do you have any ideas on how to solve these problems, or can you think of an other way on how to run the two scripts network based on the various events?

Please ask if you need more infos!!!
Thank you very much in advance!
Broesel