Obviously the first thing to do is check for Viruses, and the next thing is to check for mallware on the system. the two most causes of slow down of a system, or random restarts.
I always use mall-ware antibytes, you can get a free version or a trial version. I hope you never payed them any money for the service. Because to be honest there are plenty of things on the net free to an exact point to resolve such things.
I hate people who pray on less Savvy tech aware people squeezing money out of them when by asking for it, or advice.
It does not cost you anything, an should not.
In most cases just a sound bit of advice,and a few good Links to the software.