virtualbox.org

In case this helps someone else; it's not about how the software _should_ work (that's covered in several other topics)
I have a double handful of Windows VMs on a Mac host.
Just updated the host to 4.1.18 (seemed like it went OK); installed the Extension Pack (seemed like it went OK)
Then started through all the VMs to update their Guest Additions. Most of those went OK.

On two (happened to be XP guests) the Guest Additions failed, repeatedly, each time asking for one missing video driver and wanting a CD or URL.

So here's what worked:

Finally*, when nothing else worked and I found no solution suggested, I
-- got to the point in installing the guest where I had the popup asking for the missing video driver
-- navigated from the guest desktop to /Programs/Oracle
-- opened the directory under Oracle for Guest Additions
-- normally that just has an alias in it, but, it seems
-- while the Guest Additions is being installed, the files are all there
Where? Which one to use?

Can't help you, not sure, lots of stuff in there -- but I finally found an EXE file with a name that seemed likely correct and ran that.

Guest Addition installed correctly. Restarted and rebooted and everything has continued to work fine and all the VMs show guest additions properly now.
____________________
What did I try tyhat didn't work?
The advice given in earlier topics to others who had what might be similar problems from time to time, none of those recent, most of which added up to "the video driver should be there"
and
-- to reinstall.
----I reinstalled 4.1.18, and
----I reinstalled the Extension Pack, several times,
each time then trying the Guest Additions for the two problem VMs and never getting past the point of being prompted for a video driver it couldn't find
(I think the second of three, but memory fades).

I don't know if this is just me, reporting it here in case it's useful to someone else and a pattern might emerge.

PS, I did read the manual: http://www.virtualbox.org/manual/ch04.html#idp7254976
While I never got those instrutions to work (never did seem to be able to mount the ISO file that way) this did convince me that it was being mounted and so I thought, maybe I can open it ...

I'm sure there's a better way and someone will point it out, I hope.