My free upgrade from 7 pro worked well for one month, but has become increasingly unstable. Firstly the boot up screen before sign-in virtually freezes - the circulating dots stutter and almost stop - I do not know if it eventually boots because I press Ctrl-Alt-Delete and then it seems to re-boot semi-successfully (what am I doing when I press Ctrl-Alt-Delete - I have no idea, it was an experiment?). The system then takes about 20 minutes to become truly stable. During that 20 minutes most operations, but particularly browsing and application start-ups are erratic. Finally, I cannot power down through Windows; I have to use the Power button to turn the PC off. Does anyone have any suggestions?

I have a Dell Studio XPS i7 Q820 @ 1.73Ghz, 8GB ram, 250GB SSD.