Which parameters I have to fill-in when I am installing the fresh program.

Please, clarify your intentions. Do you want to install Pmail in another partition to access the same email host, and/or the same email (local) database, at the same time or not, etc. The more information you provide, the better we can find a solution for you.

An important thing to know is that two instance of Pegasus Mail can not access the same local mailbox simultaneously. Two instances can run simultaneously provided they each have their own local mailbox directory. Also, two instances can access the same hosted email but how you go about that depends on your ultimate goal. More information is needed.

It could also be valuable if you let us know your level of experience with Pegasus Mail.

Clarification:
Using of PM on 1 computer in several partitions with several systems; with e-mails (and settings) in 1 partition, e.g.
part. 1 with Windows 7, maps A and B; part. 2 with Windows 8, maps C and D; part. 3 with Windows 10, maps E and F;
PM is installed in map A and also in map C; The e-mails reside in map B.
I want to instal PM in map E, using the e-mails in map B.

You would just do a new install to E: and when the installer asks to a path to the root mailbox directory you can provide the appropriate path. In a running Pegasus Mail instance you can to the Help > About Pegasus Mail and then press to Info button to see the path being used by that instance for the Home and New mailbox locations. They should be the same in all instances, based on what you have described.

I think you are flirting with disaster though. Each Pegasus Mail installation stores its configuration files in the mailbox directory which means they're all using the same configuration settings. Windows 7 is no longer supported.
IERenderer gets upgraded per installation but its configuration file is in the mailbox directory, so updating it in one instance may modify the config file in a way that may break the other instances. It no longer supports Windows 7 either.

I can't envision why you are doing what you are doing. I wish you continued success doing it though.

Question: Don't know about multiple installs of Pegasus under windows.
Just used regedit to look at my registries and found 10 references groups that some of which list the path. C:\PMAIL...
So, would be interested how one migh have it working with these using the correct install??

Under Linux, each user has there own wine directory under there account, so I have no issues.
My main user uses /home/msetzerii/.wine/c_drive/PMAIL
and my root user users /root/.wine/c_drive/PMAIL

Each maps them to C:\PMAIL so no possible registry conflicts.

Example of first group.

[HKEY_USERS\S-1-5-21-0-0-0-1000\Software\Pegasus Mail]

[HKEY_USERS\S-1-5-21-0-0-0-1000\Software\Pegasus Mail\BaseDir]
@="C:\PMAIL\Programs"

[HKEY_USERS\S-1-5-21-0-0-0-1000\Software\Pegasus Mail\Command]
@="C:\PMAIL\Programs\WINPM-32.EXE"

[HKEY_USERS\S-1-5-21-0-0-0-1000\Software\Pegasus Mail\Identity]
@="<Default>"

[HKEY_USERS\S-1-5-21-0-0-0-1000\Software\Pegasus Mail\Mailbox]
@="C:\PMAIL\MAIL"

[HKEY_USERS\S-1-5-21-0-0-0-1000\Software\Pegasus Mail\Newmail]
@="C:\PMAIL\MAIL"

[HKEY_USERS\S-1-5-21-0-0-0-1000\Software\Pegasus Mail\Username]
@="Single-user mode"

[HKEY_USERS\S-1-5-21-0-0-0-1000\Software\Pegasus Mail\Version]
@="0491 32"

I don't think there is such a thing as a "correct install". Windows controlled folder access restricts where an installation will function properly so that dictates installation to a folder that is not a Windows protected folder.

I think the key is this... Each time an instance of Pegasus Mail is run it will modify registry entries in HKEY_CURRENT_USER\Software\Pegasus Mail such that their path will reflect the path where that winpm-32.exe resides. Windows then looks at these entries to know which instance of Pegasus Mail to invoke when a Mailto: link is clicked, and, the Pegasus Mail installer looks at them to know where Pegasus Mail is installed, although this gets complicated when a user with standard privileges elevates privileges to run the installer (eg: Run as administrator).

Regardless, the way I think of it is that Windows thinks Pegasus Mail is installed in the path from which it was last started by the currently logged in Windows user.

Yes, That is what I though would happen, so one would only be able to run the last installed version.
Only reinstalling to the other path would change the setup.

Think long ago, their were a lot less Pegasus entries in registry. Now found 11 groups.

No. All you need is a shortcut to each winpm-32.exe. When I was working I had three instances running all day, three shortcuts were on my desktop.

There was a time when Pegasus Mail was distributed as a .zip. You could extract it to any location, trigger winpm-32.exe, and it would work fine once configured. Of course there was no way to set it as the default mailer or for it to handle Mailto: links but I consider those capabilities as conveniences, not necessities.