"...an Adminstrator account which did not appear in the normal startup desktop and was NOT password protected;...."
This is one of XP's defaults, the TRUE* Administrator (i.e. root) has no password. This isn't quite as bad as it seems, since Administrator only appears as a logon option in safe mode and network logon as Administrator is disabled if the Administrator account has no password.
And if you've got physical access to the machine then all bets are off anyway.
SecPol (and closing unnecessary services) actually allows XP to be made quite secure, but it's a bit of a chore. *NIX on the other hand starts out pretty secure and if you want, you can lock it down tight as a mouse's ear'ole.
* Users can be assigned Administrator rights, but are not THE Administrator (i.e. root)