I went back into my newly built system to see if I couldn't figure out why the front panel mic jack wasn't working. After about two hours of monkeying around, I've emerged with a still non-functional front mic jack, but also, suddenly, both my keyboard and mouse have stopped functioning.

I can't get them to do anything whatsoever at any stage of the boot process.

I've been going nuts trying to think if I unplugged or unseated anything while attempting to figure out the audio connectors for the front panel, but the only thing I could possibly find was the power to the usb, which was perhaps slightly ajar.

I've tried both wired and wireless ps/2 mouse and keyboard, both with no success.

I've already had to reinstall windows once on this build, I'd really like to not have to do it again after getting all my data and programs reinstalled, so I'm hoping someone has a good suggestion that my tired brain has just overlooked.

Is there something I might have unplugged that would affect keyboard/mouse function but still allow the pc to boot normally? I can't think of anything, but, like I said, I'm stumped.

Try a barebones boot, try bios screen and if they come back, reconnect one at a time until you find the culprit. Once you do, uninstall / reinstall it. It's probably your sound card

I can't boot from safe mode or even get to bios ... keyboard powers but does not respond. I can't even boot from windows disk because the computer asks me to press a key to boot from the cd.

why would the sound card be the culprit here (particularly if it was functioning perfectly normally for two or three days after installation).

My 1st [and only?] idea was that the keyboard and mouse might not be working because of a resource conflict/shortage of resources [the mouse and keyboard use IRQ's and more]...
And that perhaps resetting the BIOS's configuration settings to the defaults might fix it.

What I can't figure is why the BIOS's configuration settings would change to produce a resource conflict...

Unless they didn't...
Perhaps you succeeded in increasing the number of hardware items needing the provision of resources [you fixed a faulty hardware item to increase the resource demand?]

A bare-bones boot would fix that.
Then you could go into the BIOS setup and check on the "PnP/PCI Configuration" and the allocation method.

Well I assumed the sound card as that was what you were trying to get front panel stuff working.

Did-you-barebones-boot-?

This would mean removing any front panel connectors, add-on cards, harddrives, cd-rom's, right down to ONLY mobo, CPU, vid and monitor, plus Ram, psu, keys and mouse.