XP wont start

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Hi

My Windows XP SP2 system has this weird bug:

Whether in regular start up of Safe mode (!) it begins by starting up apparently normally. When it gets to the blue "Choose user" screen (where you pick which user you are), it reboots

I can only start with last know configuration...

I guess the error (incompatibility) is in one of the last things Windows loads up.

I have run many registry cleanup utilities: no change.

I have tried RESTORE (to the last possible date): No change

I have not added hardware except for an (external) hard drive that works perfectly

???

Pretty worried.

TIA for any help

P. Naccache