I thought I would add this as the linking of account affects the username and pw to be used to log in.
From http://en.support.guildwars2.com/app/answers/detail/a_id/8994
Error Code 1038 x x x x
(example - 1038:11:4:1419:101)
"You are attempting to log in with an account that does not have Guild Wars 2 registered to it. Please register a Guild Wars 2 serial code to this account. For additional information, please visit http://support.guildwars2.com."
Please select the option below that best describes your account:
I have linked my Guild Wars and Guild Wars 2 Accounts
Guild Wars and Guild Wars 2 use the same login information.
If you already play Guild Wars, and you linked Guild Wars 2 to your account, then you must log in to Guild Wars 2 and the forums using the exact same account information that you use to log in to play Guild Wars.
Example 1:
Email or Account Name: myaccount@ncsoft
Password: myGuildWarsPW
Example 2:
Email or Account Name: myemail@email.fake
Password: myGuildWarsPW
If your Guild Wars account has @plaync or @ncsoft in the account name and you need to reset your Guild Wars password, you can use the Login Helper on the Guild Wars site.
Please remember that resetting your Guild Wars password after linking to a Guild Wars 2 account will reset the password for both accounts.