Here is a handy freeware security utility to lock your desktop when you step away for a moment. I thought it was nolonger available but I just found it at Sympatico and as luck has it, it was the last version released prior to discontinuation. When run it displays your choice of a screen capture, decoy bitmap, a built-in screensaver or a blank screen. Under Win9x you cannot stop its process via Ctrl-Alt-Del. You can stop its process under Win2k but doing so will leave the taskbar disabled. Really it's not needed under Win2k since it has its own superior desktop lock.http://downloads.sympatico.ca/tucows/preview/195494.shtml
By the way SystemCloak doesn't replace any system files but there is one dll you may need in order to use it.