Okay, when you typed in the Net User command, it displayed the existing account on your computer, they are:
Administrator
ASPNET
GUEST
HelpAssistant
IUSER_MARK
IWAM_MARK
ms support_388945a0
By looking at the IUSER_MARK & IWAM_MARK accounts,you obviously have named your computer Mark....you CAN NOT have a user name with the same spelling as your computer name
You can also view the user accounts in XP Pro by right clicking "My Computer"....click "Manage"....click "Local Users and Groups"...."users"....most people disable the Guest & Support accounts.
****Please confirm in here that there IS NOT a "Mark" username****
****Please confirm there is an "Administrator" account****
I suspect because you see a "Mark" account & a "Guest" account, you are using the "Welcome Screen" & "Fast User Switching" to log in & you most likely have renamed the Administrator account to "Mark" & the "Guest" account is enabled.
If you turn off Fast User Switching & the Welcome Screen under "User Accounts" in the Control Panel....reboot....you should get a loging window that will allow you to type the user name & password (the same as most workplace networks/schools are set up)....
Type Administrator for username
Type the password you have been trying for the "Mark" account for the password
It should let you in.....if not, try leaving the password field blank (don't even type any spaces) for the Administrator account & see if that will get you logged in.
Let us know how you make out.
Darryl