Erik:
A/C work is generally not something you can do with any success yourself. The older systems (pre-1993) all use R12 (Freon) refrigerant. It's gotten quite expensive- it used to be $1/lb, but is now over $40/lb. The age of your vehicle (same as mine by the way) is going to make this, quite likely, an expensive proposition. The most likely reason the system isn't working is due to being discharged, because of a leak. AC systems are very high pressure, and the working fluid is a gas- it will ALL leave through any pinhole in the system.
The quick diagnosis is this- open the hood, engine running, turn on AC. Does the compressor start to turn or does it just sit there? If it doesn't kick on at all, the system probably needs at least one major leak addressed, and then recharging.
If it turns on, then off a second later, that's a good sign. That means it has some charge left in it, just not enough. You may be able to simply charge the system and have it work for a while. In any case, when it's recharged you should have a fluorescent dye put in so you can track any leaks.
If the compressor DOES turn on and stay running, you may have a problem with the expansion valve as you suggested. This would require evacuating the system, opening up the line at the evaporator, and then replacing the valve and recharging the system.

Major components in the A/C system are:
Compressor (mounted on the engine, driver's side lower)
Condensor (the "radiator" in front of the real radiator)
Evaporator (under the dashboard in front of the heater core)

If anything's leaking, you should hope it's only a hose, aluminum line, or an O-ring seal. All three of the major components are quite expensive, and that's on top of the 2-3 lbs of R12 you'll have to buy to recharge the system.
Good luck with it! --
Robert Bareiss, Manager, Eurocars+, Groton, CT
Eurocars Plus Volvo Sales & Service