This happened to me just a few weeks ago when I was trying to make a slide show presentation to a group of about 100 people at summer camp, mostly kids and teenagers. I was preoccupied with getting an iMovie presentation ready on my MacBook Pro, so one of my friends was busy getting a slide show of about 100 JPEG photos ready on a Toshiba laptop running Vista. That Toshiba had been used earlier in the week to play a video, so I was not concerned about its ability to connect to the projector.
So it was showtime, and the MC introduced the slideshow, the lights dimmed, everyone sat expectantly. We plugged the Toshiba into the projector, and waited... the projector put out a big blue blank screen - "no input detected".
The silence grew uncomfortable. The room was still dark, cast only in a bright blue glow. We were in the back plugging and replugging, hitting Fn-F5, trying to get the unit to switch monitor outputs, to no avail. The little animation on the LCD indicated acknowledgement of what we wanted to do, but there was nothing happening on the projector. We motioned for the MC to stall for some time, which he did. He started leading the kids in a song while we continued to scramble in the back.
We decided to try rebooting the laptop. This seemed to work! The projector detected an input, the screen went black. A gigantic Windows boot logo appeared on the screen. The little progress bar chugged at the bottom. Kids started cheering!
And then -- BOOM! -- a great huge 10-foot-wide BSOD appears in front of everyone. The MC -- awash in bright blue again -- looks behind him and laughs. The kids start cheering. The poor guy who owns the Toshiba laptop is mortified.
Ultimately we ask the MC to move on with other parts of the evening program while we continue to work in the back. I grab my USB stick and copy the pictures off the Toshiba (after we have rebooted into safe mode) and onto my MacBook Pro, where we import them into iPhoto and prepare a slideshow there. We finish just in time, and they're ready to try the slideshow again.
I plug the projector into my Mac, and press "Play Slideshow" in iPhoto. The projector is detected, the screen fades to black, and the slideshow works perfectly.
And this is why I use a Mac.