I'm not sure if .NET exposes this API call in a namespace so here's the delaration:
Declare Function GetUserName Lib "advapi32.dll" Alias _
"GetUserNameA" (ByVal lpBuffer As String, nSize As Long) As Long
Public Sub Main()
Dim lLong As Long
Dim lLong2 As Long
lLong = 255
sCurrentUser = Space(lLong)
lLong2 = GetUserName(sCurrentUser, lLong)
sCurrentUser = RTrim(sCurrentUser)
end sub