I generally run my comp for 3 days at a strech and then give it 6 hours rest. Been doing this for over a year, all major components are working fine without any problems. For cooling I use 2 extra fans cabinet fans.
While downloading big huge files which take 12 to 15 hrs I generally keep my comp on for 24 to 36 hrs at a time , they are built for long use, More harmfull to a PC is the switching on and the switching off part, this is the time when most HDD's crash or component IC's burn. Make sure u r not using graphic intensive software like games , rendering or engineering as these can cause the CPU to operate at 90 - 100% and the GPU also around the same causing lot of heat generation. But if u have to do it, then open the side door of the Cabinet, and keep one of those small desktop at the side blowing air into the system.
It works fine for me, but If u have a Air Conditioner, and u can keep it on all the time then theres nothing like it.
RM Clock is a software which reduces the core CPU voltage and clock multiplier on-the-fly according to CPU load. This translates to less power consumption and less heat generated by the CPU. Get it at http://cpu.rightmark.org And yeah, its very easy! Everything is automatically controlled by the software! My CPU clock is 1800 Mhz (1.5 volts), but at idle, RM Clock underclocks it to 800 Mhz (0.8 volts)!