Hate to state the obvoius....
but is the coil box drain clogged?
or at least partially clogged?

If your car has the automatic climate control, then there's a kit you can get from Volvo that turns on the blower after the car's sat turned off for 45 minutes after having run with the A/C on. This is intended to allow time for condensation to complete, and the blower runs evaporatively dry out the system.

For both of my cars, I had this installed under the new car warranty; the key customer complaint is "smell coming from vents after a/c running". In my '94, the retrofit was via a chip that they added; I don't know about the '98 or if, in the newer CAN-bus cars if they simply upload new software. In short, I have no idea how much it costs, but it took care of the smell problems in both cars.

If your car's out of warranty, but you've had the complaint for awhile, maybe you can get your service manager or parts manager to work out a deal with you (cost for part + labor) or something.

-cj

Probably from buildup of mildew in your evap, try spraying some lysol into the wiper cowling vents with the blower fan on high and ventilation open. That should rid of the odor but to prevent this in the future try getting a pollen filter fitted (if U don't have one) and also opening the ventilation more often while the A/C is on.