It depends on your level of cover, I have worked with HP, Dell, Apple, Siemens and Acer systems at one time or another. Dell most recently as a field service engineer. If it's a warranty job then only a Dell certified engineer can work on the system if you want to keep your warranty. Turn around atthe most basic level is Return to Dell and it usualy takes about a week to fix. At the top end they do a next day home visit. Servers have a different set up.

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'The eye can only see what the mind is prepared to accept'