New Motherboard problem

Further to my previous questions, I gave up

(I run a charity that had a PC donated to it, which would not boot up, except in safe mode (which is why the owner got rid of it))

After trying all usual solutions I gave up and replaced the motherboard which did not solve the problem

So I put a hard drive from one of my boyfriends PC’s in, incase the problem was with the original hard drive (even though all diagnostic tests shoed no problem with hard drive)

To cut a long story short, even if I use a boot disk from Win 98, I get a message saying that windows has ncountered a problem and cannot find a fixed drive

This is despite when on boot up it says “detecting IDE Drives” and the CD and HD are listed correctly

If I don’t use a start up floppy, it says “invalid boot disk”