When you empty the Screenshots folder in Windows...

Here's how to reset the counter to "01" again....

In your Search window, type regedit to open the Registry Editor (as above) and then navigate to H-Key Current User > Software > Microsoft > Windows > Current Version > Explorer and then in the right-hand side, look for the entry for the Dword named ScreenshotIndex, and right-click on that to bring up the little drop-down menu, then click on the first word Modify to show another little menu where you can change the value to 01 (hexadecimal). When you've done that, and the value shown in the right-hand side for it shows 01, you can close the Registry Editor, because you're done. And that's all there is to it. Your next screenshot will be numbered 01.