Where to get power for Hard drive

May be the STUPIDEST question ever but consider the source (ME). Have a WD Caviar GP (WD5000AACS 500 GB) that I have had for a while wanting to use as backup drive. Had it installed once but had probs with desktop so removed it. Now that I want to reinstall it I cannot for the life of me figure out how to get power to it. Everyone says look from power supply box at top of inside tower. Only power supply that looks to be able to fit is the one going to hard drive installed with computer. How do I add a power source for the secondary drive. Have HP Pavilion a1102n desktop.

If your PSU has a spare 4-pin Molex peripheral connector, then something like this should do it:

Otherwise, if all the Molex connectors are in use, then a Y-splitter cable will give you an additional plug:

Here are other solutions: