Are you using a laptop with a touchpad? If so, touchpads are well known for unusual behaviour because we brush against it when typing. Also some systems have touchpads that interpret 'gestures' as key strokes.
I have a laptop with a touchpad which I don't use much but when I do I am forever brushing against it and end up deleting whole words or lines, or other unwanted things. I can turn off the touchpad when typing with a single button push, and that helps, but I now have it turned off permanently in favor of a USB mouse.
If it is not that, is Word set to auto-save at all?
What about the keyboard? When was the last time you turned it upside down and, (gently), shook it. If never you might be surprised what comes out.
I have no idea how this happened, whether I hit some strange control character or if this is a program corruption. I opened and was working on a document in Word 2000 (9.0.2720)on Windows XP, like I do every day for years. All of the sudden, to make a long story short, every 1-4 characters I type opens a "Save As" window. I can find no pattern. It happens with most if not all letters. It happens with "enter". Sometimes "saving as" opens another "Save As" window. Sometimes I have to ctl-alt-del to get out of it. Troubleshooting: I went to C>Documents and Settings>Owner>Application Data>Microsoft>Templates and deleted "Normal". (I tried this three times, but each new template had the same bug.) What next? Thank you.