I would tent first. Find out how long you need to wait before you can wash it down, your exterminator should be able to give you this info.
Once you've waited long enough, pressure wash with TSP. Rinse, then let it dry. This time of year isn't the best to paint, but make sure you give it plenty of time to dry before painting (otherwise you'll lock in moisture, which could ruin the paint, wood, or both). During the summer it's usually a week or two (depending on the surface and age) to let it fully dry, this time of year I'd probably wait longer than that (better safe than sorry).
If you need to sand and/or scrape, often it's easiest to do when the siding is still slightly damp. If you're concerned about lead paint (which you should be if your home is old enough and you don't know the history), you'll definitely want the siding to be wet when you sand and scrape (and of course wear a good painters mask). After you're done sanding/scraping, do a quick rinse then let dry.
If you use a pressure washer, make sure you get a low PSI one. You don't want an uber powerful one. If you're stripping paint with it, it's too strong, you're using too narrow of a wand, or you're too close. Also watch out for places where water can get in, most homes were not designed for horizontal driving rain, so any gaps or cracks should be caulked before you pressure wash (especially around windows).
Good luck with your project, and be sure to take pictures.
