Win98 is loading the drivers for the old m/board and this is causing the hang. You would think that plug and play would detect the new componants but the m/board drivers must be loaded first for PnP to work.
I have had some luck in booting to safe mode when the screen says . Let the pc boot up into sfae mode and when all disk activity has ceased do a shut down and reboot.
If you can get in your bios then perhaps turning the option on will aid the process.
If not then rename win.com and boot up with a win98 floppy and reload windows. It must go in the same directory as before (undoubtedly c:\windows) and most importantly you must use the same registration key as before because it compares this against the one in your Registry.
Once this is done you have a new copy of windows and because we didn't touch the Registry, all your old programs should still work (sound of wood being touched)