Yes, you know, re-install the OS, and if you don't know what it is, then i suggest you to do it, maybe that's why you're having this problems, the computers must be formatted at least every 6 months and as more, every year and a half to prevent critical failures in the system.
One more thing, when you format a computer, you lost all the data so I recommend you to do a back up of all you want to save, of course, in case you want to format yours.