Problem with "Winpopup" program under win2000...

I'm trying to make a winpopup-substitute program in VB and use it under windows 2000. Using the API functions i describe later in this message, i've succeeded making a program that can send messages to another copy of itself or winpopup and receive messages from another copy of itself. The only problem i have is when i try to send messages from winpopup to it.

If i have my program in PC (A) and winpopup in PC (B), i can send a message from (A) to (B) but not from (B) to (A). When i try to send from (B), i get a message (from winpopup) saying that i'm trying to send a message to a non-existing user (user has not logged in or something)!

MyApp -> MyApp : works ok
MyApp -> Winpopup : works ok
Winpopup -> win2000 messenger service : works ok
Winpopup -> Myapp : does not work!!!

I've used the following functions:
- CreateMailslotNoSecurity to create the mailslot \\.\mailslot\messngr (which is the one used by winpopup)
- CreateFile and ReadFileSimple to read the mailbox.
- WriteFileSimple to write to a mailbox

I assume the problem has something to do with the way i create the mailslot but i'm not sure what.
I've tried:
hMailSlot1 = CreateMailslotNoSecurity("\\.\mailslot\mymessngr", 0, 0, 0)

and
hMailSlot1 = CreateMailslotNoSecurity("\\.\mailslot\mymessngr", 0, MAILSLOT_WAIT_FOREVER, 0)

but nothing changed.

When i tried to use a different mailslot (i.e. \\.\mailslot\mymessngr), my application could not even communicate with another instance of itself running on another PC.

Unfortunatelly, searching the Net didn't help very much. Everybody describes the steps you have to follow but noone seems to have had the same problem i have. I know it is a Win2000 problem because under two win98 PCs it works OK. I'm sure it has something to do with security and networking because i have some file-sharing problems too but i don't know what...

Does anyone have any ideas out there???

TIA

Nick A.