Hi folks, when we did this before, we discovered a few things about trying to write within the parameters of the exercise. First, Microsoft Word comes with a handy 'word count' feature in the Tools drop-down menu. However, I have found it doesn't always count words in the same way you or I would. I keep recounting one story I wrote, and I count 99 words to Word's 100 count. Can't figure out what it's counting and I'm not.
About policing your repeats. I've found the only sure-fire method to ensure you are not repeating a word is to enter your story into Excel in a single column. This actually helps with word-count, too. Then sort the column alphabetically -- any repeats will show up next to each other. Make sure you either kept your story in a Word file or save before sorting in Excel, the sort is kinda permanent! OK, it's annoying having to type it into Excel, but it's only 100 words... :)
Now go. Be creative.
[added, 3:40 pm] OK, I highly recommend inputting your story into Microsoft Excel after you write and save it in Word!!! One column, one word per cell, leave out all your punctuation, confirms your 100 word count, then sort the story alphabetically to catch any word repeats!!! Works like a charm. What do you know! I had a story that was 101 words long, and I didn't realize, and... neither did Microsoft Word!