The local super groceries-and-more store had a bank of demo PCs running DOS, Geoworks or early Windows, and a screen saver. We used to type "format c: /autotest", the later being an undocumented MS-DOS parameter that skipped the "are you sure?" question, and left it standing on screen like that.

The sales personnel would occasionally terminate the screen saver by pressing "enter". Hilarity ensued (for us! Not for the poor sales person trying to give a demo to a customer) :)

As for programs and the clock, my school (which had HORRIBLE IT courses at the time) had caught a virus (not from me, just like that!).

I removed it for them (using F-PROT and McAfee for DOS back in the days), but they didn't trust that and called an "expert" business (the local typewriter guy), who ran exactly the same virus scanners, and changed them dearly. But they didn't trust him either (ahoy computer virus craze!), so they made me reinstall nine school machines, with Windows 3.1, Word 6, Excel 5, Access 2.0, from floppy disks, because I was apparently the only one capable of installing Windows.

I was so angry about the fact they didn't even properly thank me, I made a Turbo Pascal program that would also check the system clock, and do interesting things. I couldn't get a TSR working properly at the time, so I ended with an exe in the search path called " .exe" (the blank being ascii 255, not ascii 20, which wasn't allowed), and autoexec.bat would have loads of empty lines at the end, and then finally, the call to the program named with "ascii 255", which was invisible in the editor even if someone scrolled all the way down and was looking right at that particular line.

The program was reportedly still there many years thereafter. Figures :)

Another fun one (later in the Windows '95 era) was to create a shortcut to every program on the computer in the startup folder. Then copy / paste it about a dozen times and reboot.