I think you may be having two different situations.
You can test the drive by downloading the hard drive diagnostics from the manufacturer. But first the parity check is usually indicative of a failing ram chip. I've also seen this for heat issues. You might double check to make sure all vents and fans are clear of dust that might be blocking air flow.
Then test the ram in the system. There's a pretty good memory tester that Microsoft hands out for free,
http://technet.microsoft.com/en-us/magazine/2008.09.utilityspotlight.aspx As for the hard drive, what sort of raid setup do you have? You might have gotten an early warning if this is a raid 5 or raid 10 setup, you might have one drive failed but can replace it and be back up and running without a hitch. I do not think the parity error you got is tied to the hard drive. I would examine them as two seperate problems. I'll be glad to try and help you troubleshoot further, run the MS memory test and see if your ram is ok.