Just some comments:

• Pegasus Mail runs fine from the root of C: under Win10.  I haven't encountered any need to change permissions.  FWIW, each PC user needs full control over their mailbox directory.
  
• The splash screen flash without Pegasus Mail opening is an indication that it can't find the mailbox directories.
• You can specify the username in the command line using the -I option (eg: C:\PMAIL\Programs\winpm-32.exe -A -ms -I AA)
• Migrating from machine to machine via copy/paste works fine as long as the paths are identical.  Best approach is to do the copy, run it once to create the registry that records the run-from location, then run the installer over top of it.  The installer creates the shortcuts (you'll need to tweak) and the registry entries needed for full functionality.
• If you are routinely bouncing between two PC's and don't need Pegasus Mail as the default mail program consider running it from a flash drive.  I do this with the Pegasus Mail install I use for following the support list and the activity on this community forum.  It goes with me to work and when I travel. 

8 May 17: I tried to migrate a Pmail installation to Win10. (Blast you, Pegasus... you are one of the MAIN reasons I even USE Windows any more!!!)

I had been using Pmail on a WinXP box. Ver 4.7.2.572

Did a fresh, from scratch, install of Pmail on the Win10 box. (8 May 17). Just went with flow, used defaults.

Before running Pmail after install: Copy/ pasted the newly created C:/PMAIL folder to "C:/PMAIL as initially set up"

(Did a few other things. Sigh. Will come back to them.)

I got the "Flashes briefly, goes away" scenario. Actually... because the "other things" established my install as a multi-user system, I got "User, please"... and THEN (when I'd entered User, pressed enter) Pmail went away with no explanation.

I **THINK** the "whole" problem was as follows...

Win10 doesn't like us doing things in the root folder of the hard drive.

I used File Explorer (that better name is the ONE thing I like about Win10 after months of hassle with it) to access the properties for C:/PMAIL/MAIL

Under the Security tab, I added (a struggle in itself!... don't miss the "To change permissions (click) EDIT") "Modify" to the things the user I wanted to use PMAIL could do. DIDN'T, note, give them "Full control" (Well, not by that tick box, anyway.) DIDN'T give user full control of C:/PMAIL... left that folders permissions as they were.

...AND THEN PMAIL SEEMED TO RUN NORMALLY!!! Hurrah! Smiles!. (Of course, it is early days. Sigh. And what will tonight's changes to Win10 bring? But running for now!!)

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Other details... possibly unimportant, but just in case one or another matters....

First of all...

I regularly "migrate" between two computers. Each has a PMAIL installation. At the time of each migration, I take from where I'm leaving the current folders for the two users I have set up on the machines. Let's say the users are AA and BB. There's an AA folder and a BB folder in C:/PMAIL/MAIL. It is those that I take with me, as I migrate. When I arrive where I'm going, I backup the former state of the AA and BB folders on the machine I've just arrived at, and copy into C:/PMAIL/MAIL the AA ban BB folders as they were when I left the place I've come from. That's it... I can then fire up Pegasus, and continue my life, with the "new" computer looking like the "old" one did when I left it. (Along the way, I've built a nice collection of "how things were" snapshots with all my saved emails.)

BECAUSE I HAVE TWO USERS... not a lot of people will encounter this part of my hassles, I suspect, the command line options (don't faint... help coming) for the shortcut that launches Pmail for me need a tweak.

If you go into the properties of the Pmail launching shortcut, you'll find "Target: C:\PMAIL\Programs\winpm-32.exe -roam -A".

I had to make that "Target C:\PMAIL\Programs\winpm-32.exe -A -ms" to get my multi-user system to run smoothly. That is why I get the "User, please?" question when I boot my Pmail. As I said... for users who have no want for multi-user use, -roam -A is probably fine... and messing with it probably won't cure the "flashes and dies" behavior.

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I don't think it was important... but here's one of the other things I did...

The shortcut to launch Pmail initially had nothing in the "Start in" box. My XP machine had something there, "C:/PMAIL/Programs". I added that to the Win10 shortcut.

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I hate Windows.