Forgot to mention, Buy this at your local Autozone:
Liquid Wrench/Silicone spray lubricant (M9-14) | Lubricant/Grease | AutoZone.com
Spray it into the front and rear window guide uprights. If that doesn't do it clean out the track after removing the door panels, then spray the lubricant in those tracks, pretty easy really:
Pelican Technical Article: Boxster Door Panel Removal / Boxster Mirror Switch Replacement - 986 / 987