You have two potential problems. First, your power window slider mechanism might need additional lubrication. Remove the door panel and you will see the sliders. Use a light synthetic grease (Superlube or the like, but not spray lube) on each of the sliders and work the window up and down. However, since you may already have done this, your problem is likely in the switches. Take a look at Mike Ponte's great Volvo site for information on the power window switches: (http://billyworld.ultranet.com/~mponte/volvo/volvo.shtml)

These switches oxidize and corrode over time and will not carry the current to the motors. Follow Mike's instructions to remove the switch carrier in the door handle, then pop out the switches, then disassemble them and renew them. I part company with him in his advocacy of "resilvering" the switch contacts. A better approach is to buy some electronics de-oxidizer (Caig DeOxIt from an electronics supply house), spray it on, use very fine emery cloth to remove crud and grease, then reassemble. I used a small amount of silicone dielectric grease on the contact surfaces to keep them from re-oxidizing; I am going to disassemble them in a month or so and see how this has performed. This procedure is almost guaranteed to make your windows perform like new.