Spector is a package that takes periodic screen captures of your system. It then saves these as hidden files on your computer somewhere, and can be set up to email them to someone at set intervals. To check if you have Spector on your machine, type top into the Terminal (or load up the Activity Monitor). You're looking for a process called DriverSPD. If that process appears, someone has installed Spector on your computer.
To remove the Spector process (you need to be an admin user to do so), just go into /Library/StartupItems/ and delete LiloDS, DriverSPD, SystemStart, SystemStartState, and StartupParameters.plist. Log out and log back in, and DriverSPD should no longer appear in Activity Monitor. You'll also want to remove the snapshots/keylog files that Spector saves on your computer. In fact, if someone has installed this program on your computer, and you don't want them to know you found it, you may just want to delete certain snapshots, etc. Anyways, here's how you do it..
Launch your Terminal, and type cat /Library/Preferences/.abcd (without the quotes). A weirdly-formatted line that looks something like S??d/Applications/Applescript??Ӵ??????ę?J??0]?? that will show up. What you want is the path (in this case, /Applications/Applescript). This is the path to the folder in which Spector stores its hidden files.
Now in your Terminal, type ls -a PATH, replacing PATH with the path you just found (in this example, ls -a /Applications/Applescript). You should see a huge listing of files that look like _12893178263, etc. These are Spector's hidden files. You can remove individual files (based on their timestamps) that you don't want the program to see by typing cd PATH, then rm FILENAME, or just remove them all at once (type cd PATH, and then rm _*).
Here's a complete listing of files that Spector puts on your computer (in case you want to completely remove it):
/Library/Preferences/.msptrpref
/Library/Preferences/sptrapp prefs
/Library/Preferences/.abcd
/Library/startupitems/LiloDS
/Library/startupitems/DriverSPD
/Library/startupitems/SystemStart
/Library/startupitems/SystemStartState
/Library/startupitems/StartupParameters.plist
BONUS: To see the email address of the person who installed Spector, type cat /Library/Preferences/.msptrpref. Dish out payback as necessary.
[robg adds: Spector is a package that has valid uses in some environments, such as a school lab. Given that you'll need admin permissions to make the changes listed in this hint, this isn't a way for someone to work around Spector's installation in such places -- because such machines will (or definitely should!) have everyone running as non-admin users.]