Personaly I would use a better Antivirus than windows defender however it has improved alot in the last couple of years. I would recommend the Free Kaspersky scanner
There are some online scanners that you could try to see if you have a problem but they don't always remove any virus they find.
Your machine crashing can be down to other things than a virus overheat is a common cause, does it show a Blue scree error when it crashes?
Have a look in event viewer to see if errors or problems show there after a crash