Вот пример от товарища апплемана :
GetUserName

VB Declaration

Declare Function GetUserName& Lib "advapi32.dll" Alias "GetUserNameA" (ByVal _
lpBuffer As String, nSize As Long)

Description

Retrieves the name of the current user.

Use with VB

No problem.

Parameter Type/Description
lpBuffer String—A string buffer preinitialized to length nSize. It will be loaded with the user name.
nSize Long—A long variable initialized to the length of lpBuffer. On return it will contain the number of characters loaded into lpBuffer
Return Value

Long—True (nonzero) on success, zero on error. Sets GetLastError.

Platform

Windows 95, Windows NT

Example

Dim s$, cnt&, dl&
cnt& = 199
s$ = String$(2ØØ, Ø)
dl& = GetUserName(s$, cnt)
Debug.Print Left$(s$, cnt); cnt