Hi,
I've got a '93 850 GLT, 134K
I had the same problem the past few months. A squeal at start and if the A/C was on it would continue. A squeal like a dozen monkeys screaming! The squeal stoped with the A/C off, then seemingly any load on the electrical system like lowering windows or brakes when backing out the driveway in the morning would start it. The problem continued to get worse over time. Changed the serpentine belt and the problem continued. I found a great independent Volvo mechanic and he said the squeal could be caused by a number of things that hardly ever go wrong on a Volvo. First suspect the idler pulleys for the serpentine belt and the timing belt. The bearings might be bad. If it's the one for the timing belt, that's critical. A broken timimg belt can cost alot to fix the bent valves. Also I was told the serpentine belt can break (if it's idler and tension pulleys are bad) and snake into the timing belt enclosure and break the timing belt. In my case it was the Oscillation Dampener, a part that very rarely wears out. I put photos on my website, http://www.dtrivin.com/volvo/osdamp.htm
This part seems to bolt directly to the crankshaft and drives the serpentine belt.
Once this part was replaced, no more squealing!

Good Luck!