This is a problem I just discovered, when I have my fan on in my window (just your average window fan), the display on the monitor vibrates, the monitor itself isn't, when I turn the fan off, it stops, why is this?
I'm not talking about a PC fan but an actual fan, the one pictured. It's plugged into a different outlet then anything related to my desktop pc...a weird problem and kind of inconvenient since it's hot right now

Is it close to your monitor and is your monitor a CRT?If so then its the magnets in the motor.I had a small fan and a CRT before a few feet away and it cause a small shaking and color change.If possible try moving the Monitor farther away.

Yes, it is CRT and the fan's maybe 3' away, I didn't know that though, interesting tip, unfortunately I only have one window (courtesy of Pulte's inability to spend the money on another window), oh well, it's good to know why it happens though

your fan must put out ALOT of electromagnetic and magnetic interference then. i had a pencil sharpener right next to my old CRT monitor, and it did the same thing

Cell phones are also bad for this too.

I've seen it done with Pencil Sharpeners and I don't keep my phone on my desk specifically for that reason. As for the fan, I have no idea about that but there are two motors on it...