I intended to use Intel PIII 1000Mhz based PC for router. I downloaded ISO image of RouterOS 3.13 for Intel/AMD PC, booted and installed it to hard disk.

After reboot from hard disk, it just shows "Loading E" and halts.

I tried with several hard discs and ended up with the same problem. I checked BIOS settings and see nothing unusual.

What could be a problem?

I've had same problem.
Resolution was to change faulty memory slot.

Solved it. Problem was Ultra DMA mode. Disk used UDMA 5 naturally, when I lowered it to UDMA 2, it booted correctly.

I found it by using old disk which booted correctly, in found out that it used UDMA 2 naturally.