Laptop problems?

I bought my first laptop recently. It’s used. I really just wanted to be able to write, didn’t need to be connected to Wi-Fi or anything. So why is it then, that when I’m writing, the cursor; jumps out of the box, changes position and/or highlights stuff and erases it without me actually wanting to? This is getting really annoying.
Should I add that I was using the word notepad thingy that was already installed? It has Windows 7, and I just today bought MS Office and installed it, hoping that it would stop. But I feel like this may be something else not directly connected to the program. But then, I’m not a reliable source, so…..