Open WMM then under the 'Capture Video' panel (on the left) click 'Import video', select AVI file, click OK.

At the bottom of the WMM select 'Show Storyboard' (as opposed to 'Show Timeline'). Now drag and drop the video clips onto the storyboard in sequence. Remember to save the project at regular intervals, mine has a habit of freezing with larger files.

Once all your clips are on the storyboard you can add transitions between the clips if you wish, or just let one clip run into the next with no transition. You can also add titles and credits if you wish.

Once that's done click on 'Save to my computer' below 'Finish video' on the left. You will now get a dialogue.

Name the video for export and select a location (desktop maybe). Next select 'Best quality for playback on my computer', do this to see how big the file is at it's optimum quality (see lower panel). If it's under the 100Mb limit of YouTube you could save it as is.

If it's too big then select the 'Best fit to file size' option and enter the file size. I'd be tempted to go below the 100Mb limit, say 90Mb. Then click 'Next' to start saving the file.

YouTube can be slow, and occasionally you may get dropped, you just need to be patient and persevere.

If you get stuck give us a shout, and let us know how you get on.

Good luck.