Operating System Doesn,t Run

I have two systems with same configuration. PIII 700MHz,128MB RAM, Win 98 and Win 2000.

The Hard Disk(20GB) of one system no.1 is crashed yesterday hence I put Hard Disk of system no.2 in it which exactly same. But after the Bios screen runs the system doesn’t run the any operating system. It just halts showing this _ mark blinking on the left side of the monitor. The other old hard disk too works on system no.1.
what could be the problem ?
Thanks