Are you using a laptop? This will happen to me if I am careless when typing on my laptop and drop my hands down onto the track pad under the keyboard. If you don't use the trackpad, you can disable it in settings, or cover it with light cardboard.
I've also noticed that my cursor will drift if my wireless mouse is low on batteries.
Like Simkie mentioned, check the batteries if you are using a wireless mouse. If you're using a corded mouse...it might be on it's way out. Similarly, the touchpads on laptops can either crap out, or need to be recalibrated, which you can do yourself in Control Panel.
Also, if you're on an older Dell laptop....get used to it. I forget which models it was (in the 4-year old range), but some had a bug in them that made the cursor jump around, and there was never any fix discovered, by Dell or others. I borrowed a laptop from someone once and was plagued with the issue, and commented on it when I returned it..."Oh, THAT'S why we retired it. Right, sorry!"
I bought a new one in July, and the touchpad was horrendous. It would stop working out of the blue, and I would have to just shut down the computer to get it working again. After several calls to customer service, I had the whole keyboard replaced, and the problem persisted. Did an exchange, and the problem still persisted. I'm getting a refund and ordering another brand now.