Key loggers are software that traces the key strokes on your computer. It is done in such a way that a person using the computer is unaware of the fact that a key logger is installed on a particular PC.
1. Use updated antivirus and anti spyware
Some of the good anti viruses have capability to detect key loggers. So you should update your antiviruses regularly so that it gets updated for new keyloggers.
2. Use Anti keyloggers
There are anti keyloggers available that will make the task easy for you in finding a keyloggers. Use these softwares if you think there is a high possibility of keyloggers on a computer. The anti keyloggers that i will suggest you is Cyberhawk. Even  Kasperky does a good job in this area too. Also check anti-spy.info. More examples:
3. Search for keyloggers yourself
Keyloggers have the capability that they hide them self from the user but are active in the background. There are some shortcuts that are used to see them live on screen
For example : Ctrl + Alt + X or Ctrl + Alt + Wondows Key + X or desktopshark  etc.
4. Use special tools
KL-Detector
Freeware on demand keylogger scanner.
SnoopFree
Freeware antikeylogger that block hook based keyloggers as well as screen captures. For Windows XP.
PSMAntiKeyLogger
PSMAntiKeyLogger is a real-time protecting software which protects you against Keyloggers. No scanning is needed.
I Hate Keyloggers
Freeware antikeylogger that block hook based keyloggers. For Windows 2000 and XP.