Hi,
Welcome to the community. After reading your post I suggest you to run diagnostics on the hard drive & memory.
Please click on the below link or copy and paste it manually on the browser address bar & download the diagnostics software. Run tests on the hard drive & memory modules.
http://www.dell.com/support/drivers/us/en/19/DriverDetails/DriverFileFormats?DriverId=8V70J&FileId=2731092660&productCode=dimension-8400&urlProductCode=False
Please reply back with the result.

Rakesh R
Social Media Certified Professional
Dell Community
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