You are trying to delete a file that does not exist!
Clearing Prefetch files could be done in VB.NET like this: *Imports System.IO
Dim file As String
For Each file In System.IO.Directory.GetFiles("%systemroot%\prefetch")
System.IO.File.Delete(file)
Next file
To avoid geting any error:
Try
Dim file As String
For Each file In System.IO.Directory.GetFiles("%systemroot%\prefetch")
System.IO.File.Delete(file)
Next file
Catch ex As Exception
msgbox("Can not delete Prefetch right now!")
End Try
Well please run your app in Admin Mode (in win7 and 8)