The only really big ones I have done are in the slide show.
The old chap who I teach how to use his pc had about 1,000 slides. It would import them into movie maker on his XP pc, 2.8 AMD Sempron, but when we dragged the slides into the timeline and then told it to make a movie and save to hard drive it just showed the time left as going up and up, once it got to 4 hours I gave up. I copied the files to a flash drive and did them on my pc at home using Vista and it did the movie in about 30 minutes.
I found the only way I could get it to work on XP was to load around 150 slides at a time and make individual clips then when we had made all the clips I could import the clips back then make a movie by joining them in the program.
If you want to edit files created in WMC have a look at Video Redo Plus, thats far better and quicker than Roxios Gill