This is an oldie, but a goodie, and still works for me.
My two-year-old daugther really wants to play computer games like her big brother. However, every time I let her sit down to the computer, she would bang on the keys and move the mouse like a toy car, which usually resulted in opening files, prompting printer boxes, and any other number of right-click functions that threatened to crash the computer. Not to mention, her brother didn't like being kicked off his game only to watch her terrorize the hard drive.
I finally figured out a solution that makes everyone happy, and cost me about $15 total ($3 for a monitor splitter and a trip to a few garage sales for old computer parts.) This is also how mommy finds time to blog!
P.S. If you are looking for a great educational program for your child, check out the free games at www.starfall.com.