Before you repaint your house, you have to remove all loose and peeling paint. A pressure washer makes the job go faster, and you can do it yourself. Use a minimum 15-degree spray nozzle, with even strokes, 18 to 24 inches from the surface to avoid damaging the wood. For especially difficult areas, you can use a zero-degree nozzle but to get the hang of it, practice first on an out-of-the-way place. Allow your siding to dry at least 24 hours before repainting.
For more on siding, consider:
Guide to Exterior Siding
Powering Washing the Home Exterior (VIDEO)
Siding Roundup: 8 Options to Beautify Your Home