Sounds like your using windows and your not on an Admin account. However if you KNOW for a fact that your deffinatly an admin on your computer then ignore this post.
To check if your an admin click on
Start>Control Panel
From there Click on User Accounts
Your account should be listed there, If it says that its a computer admin then i dont know how to help you. If it doesnt say your an Administrator Account. Then click Start>Logoff
Now from the login screen (blue with users on it) Press alt+ctrl+del twice and it should bring up a login screen where you can type the name of the user. To login Type
Administrator
as the user. Hopefully you don't have a password on your administrator account otherwise this probably wont work. Now if its logged you in try running maplestory, Let it update and then close maplestory and click
Start>LogOff
from there log in to your usual account.
To save yourself the hassle of doing this everytime you need to update before you logoff the admin account goto
Start>Control Panel>User Accounts
And click on your user account
Select change account type and set it to an administrator account, This however can leave you open to taking more damage to your files if yo get a computer virus though so unless you know what your doing and have good antivirus software then don't do it please ;)
I'm still not really sure what a shadow user account is but if maplestory works after this then it wont matter =P
hope it helpd
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HaXoR_DB