What does key properties validity and trust 'None' mean???

[Bo Berglund]

We are new to GPG and we have used gpg4win 1.0.5 to install the
programs on Windows XP Pro PC:s. Some years ago we used PGP 7, but now
it turns out that we cannot get it to work on our XP-Pro PC:s so we
are trying GPG instead.

But we are facing a problem when we keys and export our public keys.
When we import the resulting ASC file from others into WinPT by
drag-drop it looks like it succeeds, but when the key has reached the
window we see that the validity and trust columns show up as None!

What can we do to change this? Our own keys are Ultimate both in trust
and validity, yet the public keys seem to lose this, why?
In one case I have validity=Full but trust=None and I don't know why
this happened....

Can someone please explain what we are doing wrong here?
Is there another step after generating a key pair or after exporting
the public key to make it valid and trusted?
Or is this done by the other side (the one importing the public key)?

/Bo
Bo Berglund