let me narrow it down for you....
I am using IBM 'thinkcenter' bought used.... its a 2003 model and i bought it in 2006 and its 2009 now.
Its been almost a year since i got PTCL DSL and since than i don't remember turning my PC off intentionally except 1 or 2 windows installations.
It runs 24/7 non stop without any problem not even a single hardware issue of any sort. In our country, electricity is a problem and you know past year there was too much of it.
It all comes down to branded or non branded systems, branded systems are made to run 24/7, where non branded PCs don't have that strength in them due to various different/some times not compatible/ "not made for each other kinda" hardware just put together to call it a PC.
SO if you have ANY BRANDED system, your good to go just remember to turn off ur LCD/monitor in case if ur not using the system.
I won't recommend any NON BRANDED system for 24/7 usage..... they simply suck!