<< Seriously now, here is a REALLY good one, you'll need about 5mins.
1. take a screenshot of their desktop
2. hide all desktop icons
3. use TweakUI, policy editor and other such tools to hide My computer, docs, recycle bin and IE (and other windows special short cuts). Its different for every OS, so you'll need a bit of research beforehand so youdon't waste time (you can use policy editor in Win9x, I know that much)
4. hide the task bar
5. set the backgroud to the screenshot you took in step1.
It'll be good for a few laughs.....everything willlook identical....nothing they click on will work. >>
We used to do that to new starters at a place I used to work.
For the rest of the pranks -
I don't know what the situtation is in the LANs you got to but all the ones I go to it is an offence to mess abouts with anyone elses machine without permission. I feel totally safe leaving my machine for any amout of time. If I ever found out anyone had been messing with it, I'd give 'em a boot up the a$$. Messing about with other peoples personal property is NOT funny in the slightest.