The best place to start is in the Event Viewer of that machine, you can get a full picture of what is going on.
You could also clarify the exact message being displayed to the Salesman.
You Receive a "System Has Recovered from a Serious Error" Message After Every Restart (Q317277)
316424 - Error Message: The System Has Recovered from a Serious Problem
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Some general things to try.
See if System Restore will get you back to a restore point before your problem with Explorer.
Try Safe Mode (Do you have this problem in Safe Mode?).
Try running ChkDsk to check your drive for errors. Right click your Drive icon/ Properties/ Tools/ Error Checking.
Run the System File Checker program from the Run Box by typing.....Sfc /Scannow in it and have your XP CD handy.
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If they don't work you could try repairing windows by running it over itself. You will lose all your windows updates but your files will be untouched.
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP (Q315341)