UNMOUNTABLE BOOT VOLUME is an error commonly given with the installer not understanding how to properly mount a hard disk.
This can be because of a corrupted file system or bad IDE cable, but most likely caused by an integrated/add-on UDMA 66/100/133 controller.
I know the 400Mhz is a slot-1 and if it is a slot-1, that means the chipset for the motherboard is probably BX.
BX chipsets had no native support for UDMA.. Just regular IDE channels. So some board manufacturer's began installing integrated UDMA controllers. Windows XP does not properly recognize all of these and has the same problem as you mentioned.
A solution would be to make sure your primary disk drive is placed on the standard IDE channel, and not the integraded/add-on UDMA controller. Windows XP should have no problems with a standard IDE controller.
If this does not solve it, try switching cables.
If that does not help, then the last resort might be to change IDE channels (one may be damaged) or to try another hard disk in case yours has somehow failed. Also, you may want to freshly format the volume before installation (if you are presented with that option). Chances are, it is freezing on the "Starting up Windows XP installation" screen, right?