You can use WScript.ScriptFullName which will return the full path of the executing script.
You can then use string manipulation (jscript example) :
scriptdir = WScript.ScriptFullName.substring(0,WScript.ScriptFullName.lastIndexOf(WScript.ScriptName)-1)
Or get help from FileSystemObject, (vbscript example) :
scriptdir = CreateObject("Scripting.FileSystemObject").GetParentFolderName(WScript.ScriptFullName)