Dell Optiplex 990 Locking Up after a few

So - I have a Dell Optiplex that is running a restaurant Point of Sale system.  Recently it started locking up after booting into windows.  At the restaurant, it would lock up about 5 minutes after booting into windows.  By Lock up, I mean I can't do anything - including Control-Alt-Delete.   I can still move the mouse.

At my house, it goes for longer... but after running it all day, i came back, and it was completely frozen - no mouse, no keyboard, just a windows desktop.

The Dell diagnostic lights on start up are lights 1 & 2 - and then 1 & 3 & 4.  According to - those lights indicate pending hardware/motherboard failure.  I have no cards plugged in (just using onboard NIC, and VC).  

I ran the Dell Diagnostic in the F12 setup menu after restarting.  It failed on the hard drive test.  I rebooted, and am running checkdisk now from Windows - but hasn't found any errors. My question...

Should I replace the motherboard or the harddrive?  I did a memory check too - also made sure they were seated properly - everything checks out.

Thanks for your help!!!
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If diags shows the hard drive failing then replace the hard drive

If all other diags pass then u need to start with the drive

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Run the disk manufacturer's diagnostic, and if it gives errors, replace it. You'll find those diags on the UBCD, and also memtest86+ which you should also run:
i would clean it out first from all dust, and verify that all fans are running ok
you can also install speedfan, to check if the temperatures are not heating up the pc
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He's run the diags(F12 on first splash screen when powering on the unit) and that's showed the drive as failing so that much is done, def replace that drive as if it failed diags that's a positive answer on that test
right -i overlooked the last part
you can - if the disk has bad sectors, often recover them with HDDRegenerator (not free)      

it recovered many drives for me
slightlyoffAuthor Commented:
Thanks everyone.  I replaced the hard drive.  It hasn't frozen up yet.
I appreciate the advice.
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