Well to start the model number of your PC is not on Compaq website.
They have a 5100 series listed. The reason is to check to see what will actually might fit.
Most 700-800W won't fit in my old HP case. Might not yours either.
OCZ and Corsair are good brands and you can usually find one on sale here. Newegg.com - Power Supply, ATX Power Supply, Uninterruptible Power Supply, DC Power Supply
Also the design is a bit different on fan exhaust on many prebuilts so you need the right upgrade.
Probably slide by with a 600W.
Need the right model # first.