Dell PC boots with hard drive error code

At boot-up I see an error code saying"hard drive self monitoring system has reported that a parameter has exceeded it's normal operating range." Dell recommends that you back up your data regularly. then says press f1 to continue,f2 to enter setup. I know I can fix this in dos mode,but I cant remember what key to push at start up,and windows will not start,gets caught in a loop trying to boot over and over again. Ways up with that?

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