I've had a couple of these problems with 850s and other cars that I've owned. When the cruise control on my 95 850 went out, it needed a new module, about $250-300 at an independent garage. I've had two cars (not Volvos) where the SRS light came on, and what was actually broken was the sensor in the passenger seat that detects whether a passenger is present. Obviously in your case, the right seat belt tensioner would be something to check--see if it actually locks up under hard braking. If so, there might be a switch or sensor in the tensioner that has worn out. SRS indicator problems are very common in ten-year-old cars--there are lots of switches and sensors that can wear out and cause a fault.

When I've had power window problems, the regulators and/or motors have needed to be replaced. Worth checking, though, to see if the switch might be worn out and not making a continuous connection. Perhaps you could pull out a switch from a window that is working and swap it for the left-front switch and see what happens.

With 11 years and 200k miles on your car, you're going to experience these kinds of annoyances. At least it's a Volvo and not a Hyundai!