How can I prevent aluminum window frames from heating up when they're in full sun?
Some ideas to try:


Paint the window frames white. This will reflect the light instead of absorbing it.  This might make quite a difference by itself
Cover the frames with XPS foam. On the inside you'd need to cover the foam with drywall or another fire barrier.
Plant a tree or bush near the window to help shade it
Replace the window with a vinyl window.  It's not as hard as it sounds and can definitely be DIY
Install an awning or trellis above the window (Thanks DA01)