[dba-Tech] Operating system not found

I've got a PC with real puzzling behaviour:
suddenly it tells me at boot time: Operating system not found.
Now this would indicate that either the HD is gone bad or disconnected,but:
The BIOS shows the HD is there.
then I started it with ACRONIS True Image.
Acronis shows the disk and contents and lets me do an image to a differnt 
PC, no problem here.
So I took a different HD from another PC which has WinXP on it.
Same problem - Operating system not found.
Yes I tried a different cable - nothing.
So I took an old HD, connected it instead of the original and started to 
install winXP on that.
What happened:
It recognized the HD, let me erase it, make a partition, format it and 
copied the files and initialized configuration.
Then it has to restart....
instead of continuing the installation, it started over. That means, it did 
not see the HD at boot time (although BIOS does show it).

now I started the WinXP install again, and when it did the restart, I took 
the WinXP CD out.
Here it should have asked for the CD, but it told me Operating System NOT 
FOUND  again.

So, it can read and write to the disk, but it won't recognise any OS.

now I hooked the disk up to IDE2 - this is where it recognized the Acronis 
disk as well as the WinXP disk,
but NO, it does not find the OS there either, although BIOS sees the HD.

to complete the tests I have now connected the CD drive to IDE1 and found, I 
can run Acronis fine.

so the problem is, it will not recognize an OS on a HD, but it will run from 
a CD, no matter whether on IDE1 or 2.

Does anybody have an idea? or do I have to replace the MB?

any suggestions welcome

thanks
Lembit