Is it bad to leave a PC on all day and night?

I have an all-in-one PC, the kind that doesn't have a tower and everything is built into the display.

Would it be a bad idea to leave it on all day and night for several days? Are there any risks such as it overheating?

If its on standby mode I don't see a big issue with it as long as it isn't running full speed and has good cooling. It is safer to turn it off though.

I've had PCs on for months at a time before, does no harm

Just runs up your electricity bill :P

yeah i have had mine on for a half a year at a time and nothing has ever happend

I wouldn't leave an all-in-one PC like that on any more then I had to. I doubt it's got sufficient cooling and trying to clean the excess dust you'll get will be a pain.

Actually due to power surging when turning on and off a system it is better for your systems health to leave the computer on at all times rather than turning it on and off :P

Amongst other things you should consider that fans will most likely wear out, if your OS isn't properly maintained and modified your hard drive can also wear out. Dust is also an issue but dust can still get in your system on or off, and arguably thermal paste needs to be replaced once in awhile ( which depends on what kind of compound you are using )

Personal Experience: All 3 of my latest computers, 1 lasted 7 years straight and was on almost 24/7 for that entire time. The only thing that ever died was a network card, and I believe it was due to lightning :P

My next computer lasted 5 years, again I had to replace the videocard due to the GPU cracking, which was because of the default manufactures thermal paste solidifying onto the heatsink when I was attempting to remove it. That computer is still running to this day. The 7 year old computer was junked as I had better computers to use :P