I need a low-cost switch (intervalometer?) that can turn on a low-voltage component for a predetermined time (say, two minutes), then off for, say 100 minutes repeatedly ad nauseum. I had my hopes on Arduino. Would you folks think this is doable for a complete newbie?
The very first tutorials that are suggested to new arduino users are to modify the existing examples to vary the flashing rate of an LED.
Jump in and go for it. I'm sure you could have you project up and running within an hour or two.
This is a nice entry level project.
As Trialex points out, you can use the flashing LED sample as a starting point. After you understand it you can expand the project to turn something else on / off in stead of the LED.
When starting out with something like this it's important to break the task up in smaller pieces like:
Make a LED flash on / off
Make LED flash with different On and of periods
Figure out how to replace LED with other device, taking the device voltage / current into consideration
You can always ask questions here, the combined knowledge and experience in this forum is incredible 
EXCELLENT!!! Thanks for the encouragement. My project is for a Biology experiment, and I didn't want to spend a lot of money. I'm thrilled!!!