While this PC is largely uninteresting, at the moment www.accurateit.com has them for $25 shipped. They are coming with 128MB RAM, ~4GB disk, and a CMOS password. I envisioned using this system as a NAS, containing my 250GB disk. Sadly enough, this is currently my largest-capacity disk, and I have it connected to my laptop (my fastest machine, a 2.16GHz Core Duo-based HPQ mobile workstation with Quadro graphics) via USB2. Just making a high-volume data transfer between that disk and my internal drive can raise CPU consumption to as much as ten percent for sustained periods of time (although it's typically more like three to five percent.) While this is pretty minimal, it's entirely senseless. IEEE1394 has much lower overhead and superior throughput, but the connector on my system is tiny and thus annoying.
The rumor over at the HP ITRC forums is that you can leave the battery disconnected (and the system removed from power, of course) for over four hours to clear the CMOS. There is a jumper inside that looks like a CMOS jumper, but it may have been used for Compaq management software which bypasses login security in Windows, judging from the contents of one of the PDF documents I downloaded from HP which pertains to this family of systems. I cleared CMOS by removing the battery for five hours. This leads to a lot of annoying loud beeping on the initial boot, but it works fine.
Now, the big downer: this system has serious problems handling large hard drives. Anything over about 120GB is just about guaranteed to be misdetected as a 6GB disk. It is unclear what kind of problems this causes with booting et cetera when using large disks. Various utilities are available for partitioning and installing windows on large disks for systems with stupid BIOS, but no such utilities are available for Linux. The system actually refused to properly boot when a 250GB disk was installed! A 40GB disk seems to work okay. Unfortunately, the system has no expansion slots, so you can't add any USB2 or 1394 slots. This makes this unit pretty much useless for a NAS (network attached storage) box. More and more I find myself wishing for some kind of inexpensive ATA-to-iSCSI enclosure that would let me eliminate all the USB and IEEE1394 connections on my storage devices - but this isn't it. If you could manage to boot the system from something other than your large disk, then perhaps Linux could be persuaded to see its true capacity?
Given its limitations, what WOULD this system be good for, today? Yank the hard disk out entirely and netboot the system. The system is reported (on HP ITRC forums) to NOT boot off of USB devices, even though some models claims to (via an option in the BIOS.) This particular model doesn't even offer the option. This makes it fairly useless as a NAS device, especially if your NAS of choice is FreeNAS. But the C500 has only one fan, which is pretty wonderful. If you remove the hard disk, the system is nearly silent. It has only a 70W power supply, so you know it's never going to draw a lot of juice. Pair it with an LCD display and you've got a very nice thin client indeed. It would be a little tired for a LTSP5 client, but you could disable encryption and pep things up that way.
I network-installed Ubuntu Gutsy on it just for laughs; not that there's a lot of hardware there, but everything worked fine. If you added a compaq-specific DVD-ROM drive, it would make a dandy VGA-output DVD player, just the thing for those people with projectors with a VGA input. It should have plenty of power to do this, but any higher-resolution stream (especially MPEG4 or similar) is pretty much out of its reach. It would make a fine music jukebox, of course. Keep in mind that an optical drive plugged into 12Mbps USB (there is reputedly one dedicated 12Mbps port, and all other ports are on one hub) can only just barely manage the 1.2MB/sec for full-quality DVD video. You are thus better off streaming the data across the 100Mbps ethernet connection; even downloading the install files for Ubuntu from my 850 MHz Pentium 3 laptop, I managed 4 MB/sec.
This system also supports wake-on-lan, and has PXE boot support. That means that you can remotely reboot this system to serve various tasks. By default, the system will boot from any attached removable device (Compaq Multibay options included optical, zip, and LS-120 drives) before the hard disk, and drop to the network adapter as the last resort, but of course the boot order can be changed in the BIOS. Multibay drives tend to be inexpensive when you can locate them, but you are unlikely to find anything much more exciting than a laptop hard drive adapter or a DVD-ROM/CD-RW combo drive. Again, whatever you're doing I suggest booting from the network, which makes this single-fan machine nearly silent and really a whole lot more interesting. Most recently I installed 128MB RAM, yanked the hard disk, and installed LTSP on a server I have here. The result is a PC that I can take anywhere and plug in to the network and power, and run anything installed on my LTSP image. This can let you use the system as a print server, WiFi extender, or for any number of other purposes.
Naturally, the real tragedy from my point of view is that Compaq didn't see fit to throw any kind of expansion slot in there. A 1xType III/2x Type II or even a 1xType III PC Card slot would allow some very nice expansion options, for example combination USB2/IEE1394, which would definitely permit this system to operate as a credible NAS. Instead, it is relegated to the thin client and audio server role. I've used USB flash devices under LTSP5, and the audio works fine, so this is probably the best purpose to which this system can be put.
Pro: Quiet, cheap, 5xUSB, WoL
Con: Slow CPU, slow memory, slow USB, no expansion