Lower voltage cables have higher current for the same power delivery, and thus higher power loss (current*resistance). Power loss in the cable means voltage loss also, so if you would try to run your 12V, 5V and so on lines from far away, you would experience a lot of power loss in those cables and would also see the voltages drop below allowed values, unless you made the cables ridiculously thick.
Moving the PC itself is a much better idea. Mine is in a walk-in closet next to my study.
The max length of usb 2.0 cabling is 5m. Longer than that and you need a powered usb hub in between (for every 5m). Do note that the mouse and keyboard already have maybe 2m of cable on them, so the extension cord can be only 3m or so. There are also solutions which use a different type of cable and have an adapter on both ends to extend the distance, but they are expensive.
For display, the maximum length of DVI cables is not really defined, but by my personal experience a 5m long dual link cable works ok on my [email protected]
For audio, you might want to use optical connections. However I just have a 5m long 3.5mm <-> 2xRCA to my amp.
You could also have an external optical drive on usb, but a better idea is to ditch optical disks altogether. Buy your games from steam so you don't have to switch disks at all. Rip your movie collection to hard drive.
One of my friends has his watercooling reserator+fan hanging outside from his window. I guess this is exactly what you were after? It seems to work fine, even though I would never use water cooling, too much effort.