What you're talking about is corrupted CMOS caused by almost-dead CMOS battery.
If replacing a battery did not help, try setting up BIOS, sence it had lost all the settings the moment you removed the battery. There is one crucial setting that can cause the "os not found error" placed in Standard CMOS Settings in your BIOS. It is the HD mode. Usualy you have a choice between LBA/Large/Normal/auto. You should set it to LBA. I imagine that is set on auto-detect (as system default), but the CMOS (that has been corrupted) also contains specifications of number of different hard drives. It is possible that, in your case, it sets the drive wong way. As a result, it's not working properly or at all. To solve this you will need to flash your BIOS.
To do so follow these steps:
- when booting you will notice a number on your lower left corner of the screen (during memory check). Press pause button and write it down. That number represents manufacturer of your motherboard and the model. Coded signature, so to speak.
On that same screen (above the mem. chk line) will be BIOS version (Example: Award bios 6.0pg). You'll need that too.
- find a copy of that exact cmos on the internet by the numbers you recorded earlier. (try http://www.bios-drivers.com/ or your motherboard manufacturer web site) use the first set of number to search the site. When you find it, make sure that the version 8or revision) match too. Download the BIOS file. Usualy the name of the file cosists of that first number you wrote and extention .BIN but don't have to be so, and the size will be 128 kb, 512kb or whatever the size of your CMOS is.
- now you need BIOS fashing utility. That depends if your bios is AWARD (Phoenix) or AMI and your motherboard model. Safest choice would be manufacturer's site (like this one http://www.tyan.com/support/html/flash_utilities.html )
There you should find the flashing utilities, CMOS file and instructions how to use the utility in one place.
- flash your cmos. Tip: DO NOT INTERUPT THE FLASHING PROCESS AT ANY COST. If you do, you will render cmos chip (and your machine)beyond all usage. (There is a cure for that also, but it's whole different story)
That should do it.
oh, yes.... regarding that mother-in-law PC... same model - same age - same cmos battery lifespan - same problems - same solution.