For what it's worth

Dell's Dimension series (4600 certainly, and I think several other models) have a crude fault-finding system that involves four illuminated letters (ABCD) on the motherboard. Each light glows green or yellow, and they change as the boot-up sequence progresses.

Mine showed green-yellow-green-green (ie A green, B yellow, C green, D green). This isn't listed as one of the codes for which an explanation is offered. In my case, (fans operating but HDD did not boot, and CPU did not warm up) it turned out to be a failed power supply. Hope this helps someone else.

Postscript December 2010: same PC, same code. Tested power supply which was OK. So replaced CMOS battery, removed memory, restarted (occasional beeps that were too far apart to be considered a pattern that could be looked up), replaced memory, restarted, and all OK. POST reported having closed down "due to a thermal event". Make of this what you can!