Manipulate the Mouse
Reader @edcomingatyou discreetly slipped a wireless mouse USB receiver into a coworker's PC and remotely interfered with his clicks and cursor movements. It's possessed!
PCWorld Senior Editor Robert Strohmeyer suggests using a wireless USB remote presenter instead of a wireless mouse. Similar effect, but you don't have to deal with potential device conflicts (some configurations don't do well with two mice at once). Also, it's more discreet than a mouse--you can even put it in your pocket and stand behind them while you mess with them.
If your neighbor is catching you with this one, play along. Pretend to call IT because your computer has been hacked and doesn't work, and take a long lunch.
Switch the Operating System
When it comes to tech pranks, nothing will take your target out of their comfort zone like swapping their operating system out for a new one. However, installing a new OS is a pain when you want to actually use it, so save your prank time by burning a Linux boot CD (Knoppix, for example) and setting their computer's BIOS to boot from a CD before looking on the hard drive for an OS. All of their data will be intact and untouched, of course--they'll just have to get their work done in OpenOffice instead. Who knows? Maybe they'll like it.
PCWorld Staff Editor Nate Ralph adds his own touch by gluing or duct-taping the optical drive door shut to prevent them from swapping the disc out. Alternately, you could opt instead to boot from a very, very small USB drive. (Bonus points if you use both the CD and the USB drive.)
On the other hand, if you're the recipient of an unwelcome upgrade, thank them for going out of their way to install a new OS on your PC, then make their Windows Vista/7 PC look like Windows XP in all its garish neon glory, or switch them to the Windows Classic theme to bring them back to the Windows 95 days. Or, if they're still running XP, just upgrade their PC to Vista. They'll forgive you someday.
Have your own favorite geeky gotchas and techy tricks? Post them in the comments. Also, read 15 Fun Ways To Pwn Coworkers for more good pranking material.
Patrick Miller is a staff editor for PCWorld. Find him off-duty @pattheflip.