Have you installed any updates lately?
Is it possible to restroe the computer to before the previous update? Windows updates will automatically create a restore point where you can roll the system back to how it was before the update.
Have you changed any of the BIOS settings to try and improve performance?
Have you installed any new devices on the computer, even a new mouse?
Have you installed and updated graphics driver? These can cause exactly the problems you mention especially if downloaded from the graphics card manufacturers website rather than the computer manufacturers site.