Hi guys,
I was going to suggest what David did, especially given that you've used Composer before with apparently no problem. Especially if Windows does its "archive/compact/rearrange/defragment" thing(s), it can sometimes mess up software.
You can visit
www.notation.com/Redownload.php any time to have your download link emailed to you.
If a reinstall doesn't fix the problem, and given the good info that you've given about your situation, a couple other "maybes" come to mind:
1. The "s" key on your keyboard may be "twitchy" (this happened to a key on my kids' computer keyboard one time)
2. If you go back and forth between a couple of different machines but are using the same keyboard, the software that does the switching may be sticking. A friend of mine used to have this problem as well.
Let's hope the reinstall fixes the problem
ttfn,
Sherry