Yeah, I'm in agreement with the other replies, they can't possibly have the same motherboard, the 300 is a Socket 7 CPU and the P2 400 is a Slot 1 CPU.
Here's what I would do.
0.Make sure the Windows CAB files are on a folder on the Hard drive that is going in her "new" system. You may also want to copy any drivers to a folder on the hard drive as well.
1.Swap the hard drives.
2.Boot the system into safe mode.
3.Run regedit.
4.Open up the HKEY Local Machine
Delete the whole Enum branch.
5. Reboot. Hit esc when it complains about the video driver. It shouldn't pop up with detecting any new hard ware just yet.
6. launch start menu, settings, control, panel, add new hardware. Do the full detection. Let it finish, you may have to point to the CAB files which are on the hard drive if you followed step 0.
7. restart the system just like it wants.
7.5. if it detects any hardware, be sure to point to to appropriate drivers. be aware that you may/should not have a cd-rom drive at this point.
8. open system properties and goto device manager. There may be two video devices, remove the one WITHOUT an exclamation mark. There may be two keyboards, remove the one WITHOUT an exclamation mark. There may be two Direct Memory Access Controllers, remove the one WITHOUT any exclamation mark. Reboot the system.
8.5. if it detects any hardware, be sure to point to to appropriate drivers. be aware that you may/should not have a cd-rom drive at this point.
9. Anything it didn't install a driver for, update/change the driver through device manager now.
10. If the system is indeed a BH6, it shouldn't need any extra drivers. If it's a VIA motherboard, I recommend installing the latest non-beta VIA 4 in 1 drivers.
11. You're done. All the software and programs should be quite content. In the worse case, you may have to check some apps over real quick that utilize the modem, as it may a blank entry or invalid modem selected. (common problem, especially for any dial up networking connections, just check the properties, say ok to the error, and reselect the correct modem)