PC PCU and common ground

Good afternoon, I'm planning on powering my arduino from the 12v rail of a PC power supply. I'll be powering other stuff (just a servo, intially) from the 5v rail.

To have a common ground will I need to use the same ground pin, on the same power chain from the PSU?

I know from experience that the different power lines [by lines I mean the physical branches of power leads coming out of the PSU] coming out of a PSU are somehow separated (if I suddenly add load on one, something else on that line might stop working. Doesn't happen to devices on other lines) but I've no idea whether that's just because of serial/parallel difference or because there's something more complicated going on. And, more importantly, whether that means the grounds are on different lines aren't common.

Thanks. I hope this makes sense, I seem to have rambled a bit.

The grounds should all be common. If they were not, I would think there would a lot of problems inside your computer.

If the power lines are somehow separately regulated or fused, it would most likely be on the positive side.

All grounds on a PC power supply are common.

For why you need common grounds look here:- http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

I've read that page, grumpy mike, that's why I knew to ask :)

I wasn't sure it needed it inside a PC as the parts that are powered separately could quite easily cope with separate power supplies (presumably by sharing ground via the connecting cable, or isolating the communication cable. I dunno.

Anyway, thanks for your answer :)