Calling VC++ DLL in VB passing STRING

Ok yet another road block. I have a simple DLL written
in VC ++ version 6.0. Lets take this for example..

long _stdcall Login(LPCSTR server)
{
..login to server
return 123;
}

Now if I call this function in VB.NET more then once from
a test app I get an "abnormal program termination" error
on the second Login call. If I run this from a VB6 app
things work perfectly. If I remove the server parameter
from the VC++ function then it works great in VB.NET but
I need to know the server to connect to.

The Function in VB is...
Public Declare Function Login Lib "testDLL5.dll" (ByVal
aServer as string) As int32

Please help !!

-Michael