system hangs after logging off and logging back!

Work through these Troubleshooting Windows 7 Failure to Start which will gather clues and help you become more familar with the system's resources.

If this is still the corrupt Dell preinstall then I would strongly consider getting a perfect Clean Reinstall - Factory OEM Windows 7 to shed the bloatware which throttles Win7 so you can't enjoy its native performance.

At the minimum I'd Clean Up Factory Bloatware.

Since Dell has built-in Diagnostics I'd run them now to clear the hardware: Dell Diagnostics Utility - How to Run the
 
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