|"....Desktop won't boot"|
You can't boot all the way into Windows is your problem.
I'm assumimg there is only one Windows installation on the computer. If that's not your case, tell us.
Your mboard is booting fine. The cmos battery doesn't even have to be installed for the mboard to be able to boot. There's nothing in the default bios settings that were loaded when you removed the battery that prevented the hard drive from booting.
"STOP: 0x0000007B (...)" This code means Inaccessible Boot Device,"
That's a message that can only be generated by Windows itself. You would not be getting that message if the hard drive was not trying to load Windows.
That indicates the bios has found a bootable hard drive, and that hard drive has Windows on it, and Windows has tried to start to load.
You can get that error you're getting if there is a ram problem.
A common thing that can happen with ram, even ram that worked fine previously, is the ram has, or has developed, a poor connection in it's slot(s).
This usually happens a long time after the ram was installed, but it can happen with new ram, or after moving the computer case from one place to another, and I've had even new modules that needed to have their contacts cleaned.
See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated:
For a brand name computer, see the Owner's or User's manual if you need to - how to remove or replace the ram is usually in that - it may already be in your installed programs. If you can't get into Windows, it may be on a disk that came with the computer, or you can go online and look at it or download it - it's in the downloads for your specific model.
You can get that error you're getting if any drive's data cable has a problem.
It is common to un-intentionally damage IDE data cables, especially while removing them - the 80 wire ones are more likely to be damaged. What usually happens is the cable is ripped at either edge and the wires there are either damaged or severed, often right at a connector or under it's cable clamp there, where it's hard to see - if a wire is severed but it's ends are touching, the connection is intermittant, rather than being reliable.
Another common thing is for the data cable to be separated from the connector contacts a bit after you have removed a cable - there should be no gap between the data cable and the connector - if there is press the cable against the connector to eliminate the gap.
80 wire data cables are also easily damaged at either edge if the cable is sharply creased at a fold in the cable.
Try another data cable if in doubt.
Check your SATA data cables. The connector on each end should "latch" into the socket on the drive and on the mboard, or on the drive controller card - it should not move when you merely brush your hand against it near the socket - if it does, mere vibration can cause a poor connection of it - use another SATA data cable that does "latch", or tape the connector in place.
(There is a slight projection or bump on one side of the outside of the connector that "latches" it into the socket - it's easily broken off or damaged)
The same thing applies for the SATA power connection.
You can get that error you're getting if the hard drive is failing, and files are corrupted or missing on the hard drive because of that.
Check your hard drive with the manufacturer's diagnostics.
See the latter part of response 1 in this:
(thanks to Dan Penny for this link:)
Hard Drive Diagnostics Tools and Utilities
If you don't have a floppy drive, you can get a CD image diagnostic utility from most hard drive manufacturer's web sites, but obviously you would need to make a burned CD, preferably a CD-R for best compatibilty, on another computer if you need to.
If the hard drive itself tests okay, any data problems found can be fixed, one way or another.