No matter how hard I tried to lift, the window would not raise. Turns out, it got stuck after the house was painted recently. Apparently they closed the window before the paint was 100% dry, therefore the window became semi-glued shut.
I went to the tool drawer, got a hammer, and a long flat head screwdriver. Tapped the sides of the window frame lightly with the hammer, and pried around the edges of the frame with the screwdriver. After fiddling with it for about 10 minutes, FINALLY got the window to raise. Sprayed some WD-40 on the glider to loosen things up a bit more, so it wouldn't get stuck again.
After wrestling with that window, I went around the house to check the rest. No problem with any of the other windows, it was just the one in here by the computer.