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I strongly recommend Bios password otherwise Its quite easy to override XP password protected details (not disclosing details here how to do it) if sitting in front of your PC . Of course I'm not your averange user But I have been called appon by some companys to log into There NT servers that there Last IT guy changed the password to (that reminds me they still owe me a beer).
But the best thing to do in my book is have the PC either always in sight when others are using it or have it locked away and only you have the key to the room its in.