Pass it the Account name of the you're looking for, it will return TRUE is the user exists. sDisplayName will hold the users Display Name.

eg:

if UserExists("dave",sDisplayName) then
  wscript.echo "Found " & sDisplayName
else
  wscript.echo "Not account found for 'Dave'"
end if
 

Function UserExists(sUser,sDisplayName)
  Dim oConnection, oCommand, oRoot, sDNSDomain, sQuery, sFilter, oResults
  UserExists = False
  sDisplayName = sUser
  On Error Resume Next
  ' Use ADO to search the domain for all users.
  Set oConnection = CreateObject("ADODB.Connection")
  Set oCommand = CreateObject("ADODB.Command")
  oConnection.Provider = "ADsDSOOBject"
  oConnection.Open "Active Directory Provider"
  Set oCommand.ActiveConnection = oConnection
  ' Determine the DNS domain from the RootDSE object.
  Set oRoot = GetObject("LDAP://RootDSE")
  sDNSDomain = oRoot.Get("DefaultNamingContext")
  sFilter = "(&(ObjectClass=user)(ObjectCategory=person)(samAccountName=" & sUser & "))"
  sQuery = "<LDAP://" & sDNSDomain & ">;" & sFilter & ";displayName;subtree"
  oCommand.CommandText = sQuery
  oCommand.Properties("Page Size") = 100
  oCommand.Properties("Timeout") = 30
  oCommand.Properties("Cache Results") = False
  Set oResults = oCommand.Execute
  Do Until oResults.EOF
    if oResults.Fields("displayName") <> "" then
      sDisplayName = oResults.Fields("displayName")
      UserExists = True
    End if
    oResults.MoveNext
  Loop
  On Error Goto 0
End Function