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ESCD Pool Data in conjunction with PnP capable operating system (Windows 9x / 2000 / ...)

On machines with an ESCD the OS may insert references to non-PnP devices without your knowledge ! This is, of course, to help the BIOS come up with a 'better' working boot configuration (by helping the BIOS know about used resources that the BIOS couldn't normally detect). This explains why (A) re-booting a non-working Windows PC sometimes makes it work (and also why Windows wants to reboot all the time when adding new hardware), and (B) also explains how those entries got there in the first place.

Windows removes entries from the ESCD when you delete a device from Device Manager. However it won't remove ESCD entries it doesn't have device manager entries for. Thus, the ESCD can be left in a state where 'false' entries remain if a non-PnP device is removed from the system and, say, Windows is deleted, re-installed, etc at the same time without actually deleting the existing non-PnP device from device manager first.

Some BIOS's have a "Reset Configuration data" CMOS setup option to erase the ESCD and 'start over'. In this case, USE IT !
If the BIOS doesn't have such a function, you are in trouble.

Why does this happen?

It's a Catch 22 situation. Windows only alters the ESCD according to changes made in Device Manger. The BIOS only updates PnP info in the ESCD, not non-PnP info. Windows trusts the BIOS to have the ESCD info 'right' for devices it does not have listed in it's device manager registry, and similarly the BIOS doesn't delete any non-PnP entires other programs put there, as it is trusting that those devices really do exist and need to be taken into account.

Each is trusting the other and nobody is doing the housekeeping. Remember, this only applies to non-PnP devices - namely Legacy VESA & ISA devices - it can never apply to PnP compatible devices.