It depends on which diagnostic you ran -- the initial one takes about 10-15 minutes and does a cursory test.  After that there's an option for a surface/extended test -- this is the one you need to run.  If it's a drive of hundreds of megabytes or a terabyte or more, this test will take several hours to run.  If the drive passes that, you will almost certainly need to reinstall Windows -- if it fails, replace the drive. Even if the drive does pass, if it's original to the system it's over a decade old -- replacing it before reloading the operating system would still be wise to do.  
My system (Del INSPIRON N5030) is not booting It's telling me to reseat the hardware and I've tried all the solutions provided here for other complains similar to mine but it's still not working! Please help me, what do I do?
My system isn't booting due to hard drive