As was already suggested, you could download the demo for another software program and use it temporarily.
Another option is that you could get another computer, and then put the SP demo on that computer.
Another option is to reformat your computer (if it's new, this hopefully wouldn't be a big problem) and then put the SP demo on it.
Of course, make sure you backup/save any important files on it first!
One other possibility might be changing the date on your computer back to before the trial expired. However, depending on how SP's demo protection is written, this might also prevent it from functioning.