In that case, I wouldn't use the batch file method - using external applications is never a good idea when it's just as easy to do it yourself.
This code may work - I haven't tested it:
Dim PrefetchDir as DirectoryInfo = New DirectoryInfo("%WINDIR%\Prefetch")
Dim files() as FileInfo = PrefetchDir.GetFiles()
Foreach file In files
file.Delete()
End Foreach