@Gill - I've been facing the same problem with stale air (it sure stinks) when only the blower is 'ON' and I end up lowering the window or turning on the AC immediately. I've got so used to it that it's now a part of my morning start-routine which goes something like this ...
1) Turn Ignition
2) Wear seatbelt
3) Crank the engine
4) Check OSVM's
5) Reverse the car
6) Turn the blower on ... *take a deep breath* ... hmm ... *satisfied grin* ... nothing like stale air to kick start a day
.
This gets nasty when there are other passengers in the car though coz they have this look on their face which says 'dude, and jus what have you been eating?' ...
On a serious note, as suggested here, I think it must be some kinda fungus (with a nasty stink he! he!) which is kinda inclined towards swift's. Are you planning to clean the vents?