Oy. This is a tough one to help with remotely. It does sound like the keyboard works some of the time, vs. almost never which is what I inferred from your op. At least it works enough to forkle your installed system, eh?
So, neither keyboard worked with either your original XP or your Dell setup disc, right? And you powered off between switching them and only had one attached at a time?
I don't want to get ahead of myself and some of the details are important. We could restore factory BIOS defaults & such but I'm not confident I understand the problem well enough yet to start throwing kitchen sinks much further.
You said you can navigate the sys setup menu for boot order. Is that the F12 boot menu or something else? Can you enter BIOS setup with F2? Don't worry, you can just reboot without doing anything.
If you want to try the reset - can't really hurt - you'll need to go into the F2 setup menu, then do an Alt-F or F9 or somesuch, depending on the Dell. Save changes, unplug the power, hit the power button to discharge caps, whistle a tune, plug it back in and see what happens.
In this context "CMOS" is sort of a catch-all nick for several types of persistent storage used in computers to remember things like hardware configuration and preferences. When corrupted it can cause anything from an apparently dead system to random crashes. Resetting or restoring to factory defaults means different things on different systems. If the system is alive you can tell it to reset itself, like we're attempting above. Other times you have to resort to pulling the battery, shorting a jumper, applying voltage to bad places at just the right time, etc.