As for it reading 100MHz next to the 350MHz for the CPU details, I believe that 100MHz refers to the clock speed.
Almost Rodders
The 100MHz refers to the BUS SPEED. This speed setting sounds correct for a 350Mhz. The CLOCK SPEED for your processor will be "3.5x" --- 3.5 x 100Mhz = 350Mhz, so I think all is well there.
It sounds like there may be a boot sector problem, or boot.ini file problem. Download the HDD diagnostic software from the HDD's manufacturers website, create a bootable floppy, and use that to make sure your HDD is ok.
IF you were to take another HDD (With Windows on) and put it into this computer, the chances of it working are slim - because the hardware is different (isn't it?) You said you checked it on another machine.... which is strange....
...but if that is the case, then it would point more at the mainboard causing an problem.