OCTools.com has an article about a couple of guys who supercooled a computer using Flourinert and liquid nitrogen. They were able to overclock a 566MHz Celeron to over 1.0GHz before their CMOS got corrupted. This is a follow up to an earlier article where they overclocked a 366MHz Celeron to over 650MHz.
It should be noted that this system is entirely fanless, though the PSU is external. Additionally, with a little more Flourinert, you could run a top-of-the-line GeForce 4 card and a 2.2GHz P4, all without any fans. Now, given the cost ($1000 for the Flourinert alone!!) and danger (liquid nitrogen in your lap, anyone?) of building this system, I'm not sure this is a reasonable desktop PC. It is, however, a very cool system.