Are you using a laptop.Maybe when you type you are accidentally touching the mouse.This can also be a driver problem but it would not only effect Microsoft word but you whole computer.If you are using a desktop try unplugging the mouse and pluging it back in.What version of Word are you using?Let us know how it works out!

I remeber having this problem with one of my computers.I just replaced the mouse and it worked.I didn`t not check if he mouse needed cleaning,maybe there was diret inside.Try replacing it.

your most likely a retard... but if thats not the case, then your mouse is broken

There are two possibilities on the table. These possibilities are either your mouse is broken or you are completely retarded with bad hand and eye coordination.

lets be nice.This is not a bad question.

I've had this happen to me as well using laptops. It turned out that when typing my hand would sometimes graze the touch pad. The first few times its happened to me I was stumped as to why because it was so subtle. Maybe this could be your problem. Also, this might seem silly, but have you tried restarting your computer?