Well, off hand, I'd do it with a Logitech QuickCam Express (i.e. the original cue ball webcam), since its cyclopean eye is staring back from the top of my monitor, as I type.
Fortunately for you, you have a PII, which can handle the camera, provided that it has USB. (This shouldn't be too difficult to add, if it has a built-in header, or any PCI slots available.)
Logitech has an
SDK* for this, allowing camera objects to be embedded as ActiveX controls in programs. I've done it with an older version of the SDK and a Delphi 6 program--but that was a couple of years ago, in high school. With a little work, I think that it's possible to record an uncompressed video stream to a file. It would then be a matter of creating a new file every few minutes, and deleting the oldest one (so that you have several consecutive minutes of "tape" before deleting any).
If that's too much (because you'd need SDK reference to figure that out easily), then
Media Player Classic (finally a new version--6.4.8.3) has the option to record video (and sound). In fact, I can get 10 frames per second at 640 × 480, or 15 frames per second at 320 × 240 (or several other low resolutions). Depending on the compression options, it may be possible to record enough in one go to last several hours. Don't try this uncompressed, though--I just got 23 MB per second.... Recording as an .avi with DivX compression, and with a brief adjustment of settings, I got a half-decent 1 min video that only used 348 KB. Since it will stare at an empty hallway most of the time, the bitrate will be low until a victim crosses its path. You can even store hours of video on a small hard drive, this way.
And if you're wondering whether it will shoot through a peephole, well, I am too. But it works just fine through a 1 cm hole at short range--see attached.
So, in summary, the absolute-cleanest way would be to code a program. The easiest is to use Media Player Classic. Tape the camera to a cardboard frame, stick the frame to the door using your choice of removable tape (at Waterloo, we get a sheet of "Magic Removable Mounts"--these are good). Plug it in and run it. When you hear the evil knocking, simply stop the recording, and cue through it to the appropriate time. [
Edit: Now that I think of it, the old-style QuickCam Express has a 1/4-20 thread for a tripod--you could even mount it elegantly to a bracket. No big deal for a robotics-person.]
Now who says mechanical engineering students can't come up with good ideas involving lots of software? And yes, I know that I take too long to compose posts, and therefore many, many people got in ahead of me.
*This may not work! I haven't tried this version of the SDK, only an older one.