I'll see if I can explain. Originally the HDD had a single install of XP, on the c:\ primary partition.
Then I installed another XP on the same HDD in a logical drive, drive f:\. Each could be booted and each saw the drive letters as stated.
XP doesn't care, it's quite happy to have another drive letter rather then c:\ for its system partition.
It takes care of all the symbolic references, they resolve to the proper drive letter depending on which XP is booted. For example, when XP C is booted, %appdata% resolves to c:\Documents and Settings\userid\Application Data
and when XP F is booted %appdata% resolves to F:\Documents and Settings\userid\Application Data.
%windir% resolves to F:\Windows when XP F is running, etc, etc.
Then I installed windows 10 in another logical drive, same HDD. 10 insists that its system drive, as known by itself must be C:\.
When 10 is booted, it sees it's own system drive as c:\, F remains F and it sees the other XP as d:\.
This doesn't make the slightest but of difference to the XP's - when they're booted,
their drive letters are still good, as each sees it. The drive letters used by a particular OS install are stored
in the registry for each system (and they don't have to be the same).
I've had it this way for years, it works perfectly.
In any case, this recent problem had been duplicated in both XP 's.
These old avast installations both expired on the same day, which seems odd,
but it could be just a coincidence.
Disk 0 is data only, but it also has the Linux Grub boot loader in it's boot record.
Disk1 contains both XP's, Windows 10, the 10 boot loader and a Linux system at the end (no drive letter from within XP).
Notice that from within XP, it sees the Windows 10 partition as k:\
Disk 1 is the default boot disk as designated in the BIOS, so if I turn on the PC it boots directly to Windows 10
via 10's boot loader.
If I want to boot either XP or Linux, I hit F11 during the BIOS POst, and select disk 0 from the menu, which invokes GRUB.
From there I can choose XP (either) or LINUX.
It may be unconventional but it works great.
(disk layout as seen from XP F)
![](https://i.postimg.cc/67fnfHh2/disk.jpg)
And this is the disk layout as seem from within Windows 10, notice some different drive letters
![](https://i.postimg.cc/gnvLBR8p/disk10.jpg)