Could you please indicate me the steps you use to install the Guest PCs?
If you are asking me, the steps I followed were traditional ones, i.e. similar to the ones I used to install other VMs (Win10, Android).
1. I had a Iso WinXPSp3 (32bit).
2. I Setup initially the VM, chosing the obvious options for the Guest OS I wanted.
3. I installed WinXP, with no extra users, similar to the installation I had in a working old laptop.
4. I changed the Virtual CD Drive from WinXP to Guest Additions (GA).
5. I tried to install GA (with generic installer and with x86 one), and I had your problem (my host had a separate video card, if this matters).
6. After having searched in this forum and elsewhere if the problem was known (at that time I did not find much), I started tweaking the settings (see the previous image).
7. I tried also to install the VGA controller driver myself from Intel and other sources, seeing that it was with a question mark in hardware resources (I did not know that it should be installed by GA). I did not succeed.
8. I try again to install GA (generic installer) and eventually I got the "right" warning to install no-signed drivers. I said eventually, because it was at this point that I waited, since also to have the warning of point 5 there was a time lag.
9. I said yes to the installation without signed certificate and at the end I checked the VGA driver in hardware resources: it was ok (see previous post).
10. I did not install the extension pack at this time. To be fair I do not even know what it is for. I just notice that there is a folder with that name in VB: so I installed it (same version) with VB itself last year.
I hope it helps.