The following are some useful resources on PC Power Supplies and troubleshooting them:

• Power Supply Fundamentals
• A Guide To Computer Power Supplies
• Power Supply Fundamentals
• Checking your PCs Power Supply Voltage Readings (good)
• PSU Troubleshooting with a Multimeter
• Computer Boot Problems
• Upgrading to a High Efficiency Power Supply
• Real System Power Requirements
• Power Supply Connectors

A few tips on buying a PC power supply:

• When Hardware is Free Power is Expensive
• Recommended Power Supplies
• Why estimate when you can measure?
• Power, Surge Protection, PCs and You
• 80 PLUS program for efficient power supplies

Some very important points from these articles (these are direct quotes):

• Most desktop PCs barely use more than 200w of power
• Power efficiency peaks around 250w
• Your PC is statistically idle 99% of the time it is turned on

To help remember which wire is which rail, think alphabetical order:

• Black = ground
• Orange = 3.3v
• Red = 5v
• Yellow=12v

Remember, black is always ground. On your multimeter red = Live, black = Ground. Always ground the multimeter with the appropriate ground connection! This is very important.

Testing for a dead power supply:

• Check power supply to motherboard connection
• Check power supply to drives
• Below 11.5V for VDC (yellow) indicates issues
• Below 10.5V for VDC means that the PC probably won’t boot.