Just off the top of my head the following all need cooling of some sort...active or passive:
CPU
Vid chip/card
RAM
Southbridge
HDD's
Interior of case (ambient)
And of course...the Operator.
I like the last one
Adding to that list, voltage regulators (VRMs) and MOFSETs can also produce a lot of heat.
If I was going to be really picky, but not over the top, I would also add that individual chips on a motherboard also do produce heat, I've experienced several boards where onboard LAN chips, sound chips, etc can get quite warm without at least a small amount of airflow.
Wow, hdd's too? I knew they generate heat but i didn't think it would up to any thing significant. So are liquid cooled computers a combination of waterplating and fan? It would seem kind of hazardous having multiple tubes through the computer for each part.
HDDs can generate a fairly significant amount of heat, depending on the spin speed.
Liquid cooling always involves fans too. The liquid would be used to take heat from the components, and is then pumped through a radiator where a fan cools the liquid back down. From there it is recirculated. That's a simplistic way of explaining it, but it is basically correct.
A "fully" watercooled machine would use water cooling on the CPU, graphics card, northbridge + southbridge, and possibly the MOFSETs. Other parts, such as the chips I mentioned, HDDs, etc, would be cooled with air still. I'm generalising here as I'm sure some people out there have literally cooled every single component that produces heat, and there are variations such as only water cooling one component.
As for "hazardous" as long as the watercooling setup is properly leak tested first, there's no real danger to watercooling. As with anything, as long as whoever is doing it understands what they're doing first, they should be fine.
Hope this helps
