Answer from other guys, It helpful to other coder.
protected override void OnStart(string[] args)
{
string[] drives = Environment.GetLogicalDrives();
_watchers = new FileSystemWatcher[drivers.Length];
int i = 0;
foreach (string strDrive in drives)
{
FileSystemWatcher _watcher = new FileSystemWatcher();
_watcher.Path = strDrive;
_watcher.Changed += new FileSystemEventHandler(FolderWatcherTest_Changed);
_watcher.Created += new FileSystemEventHandler(FolderWatcherTest_Created);
_watcher.Deleted += new FileSystemEventHandler(FolderWatcherTest_Deleted);
_watcher.Renamed += new RenamedEventHandler(FolderWatcherTest_Renamed);
_watchres[i] = watcher;
watcher.EnableRaisingEvents = true;
i++;
}
}