These instructions are for Windows Millennium but, despite Microsoft's unerring ability to make things more complicated every time they update Windows, they should work in XP and Vista as well:
Right-click on the desktop.
Select 'Properties'.
Click 'Screensaver' tab.
Choose 'My Pictures Screensaver' from the pull-down menu.
Click 'Settings'.
Choose your required time interval for each picture to be diplayed. Also choose the size and other required options.
If you want to use pictures from a folder other than 'My Pictures', click 'Browse' and navigate to the required folder.
Click 'OK'
Select the time before the screensaver cuts in.
Click 'Apply'.
Click 'OK'
Chris