I'd ask for your system specs but since this is only when starting a sketch in a Part I'd guess the problem is a corrupt Part template. Is this a custom saved Part template, or the default one from SW? And if it's a custom template have you tried starting a new Part with the default SW template to see if you still have the same issue?
Use SOLIDWORKS Rx from the start menu to run diagnostics to see if your card is supported and if so, if you are using the suggested driver.
My guess is that you are using a Geforce card. If so, it is not officially supported and we have heard of very similar problems from other users and especially that a Windows update has changed drivers that has caused the problem.
This thread has information from other users including drivers that have worked for some people.
But, your best approach is to get a graphics card that is officially supported if you are using an unsupported one.