Google around a bit and you'll find a few slide rule simulators out there. There's Slide (EXE for Windows), JavaSlide (in Java) and The Slide Rule (also in Java).
The hardest part was scanning the ruler, making sure it was perfectly horizontal, then tweaking, undistorting, redistorting and generally hacking the images to make them fit together and not look like crap. Even then, there's still some distortion and some of the scales are misaligned slightly.
LINK: YAVSR: Yet Another Virtual Slide Rule
INSTRUCTIONS: Just click and drag anywhere on the rule to move the different pieces. To switch to the opposite side of the rule, click the Flip to Other Side button.
COMPATIBILITY: Works with current versions of Mozilla, IE and Firefox.
TECHNICAL NOTE: I commented out line 1197 of wz_dragdrop.js,
"dd.obj.maximizeZ()" so that the images would maintain their original Z-order when clicked.