Sounds like something's amiss with your car. My two 850s are heroes when it comes to defrosting power. Several possibilities come to mind but the first two are likeliest:

1) Air control is in recirculation mode. Make sure you're getting fresh outside air. Other option is to use the air conditioner with recirc air. But if you've got A/C problems (definitely a Volvo weak point), it won't help.

2) Thermostat isn't working right so coolant doesn't get hot enough to heat up the air. How warm is the air leaving the heater? What does your coolant temperature gauge indicate? It should be steady at mid-range.

3) Blower fan isn't delivering much air. Mine both blow hard. Bit noisy, though, at the top 2 speeds. Others have reported that blower motors fail often.

4) Damper isn't directing air to the windshield. That's easy to tell but a long shot.

5) Heater core is plugged. Something got in there. We've heard of mice building nests. This would show up as low air flow even with the blower operating properly.

Don't give up! There are things to fault Volvos for but I don't think this is one of them.