I don't understand...do you need help accessing the administrator account on your friend's PC, because you want to install Counter Strike?
Also, what edition is it...Home or Professional?
The admin account is not accessible in normal mode in Home. However, you can access the admin account in normal mode in Windows XP Professional, by pressing Ctrl+Alt+Del at the Welcome Screen (login screen)...
With Home Edition, you need to boot into Safe Mode in order to access the Administrator account.
With both editions, the main admin account username is Administrator, and the password is whatever was specified upon installation of the OS (depending on whether one was specified).
Once in the Administrator account, you'll be able to access User Accounts setting in the Control Panel, to change account types. In your case, changing the limited account to administrator account so that you are able to install programs.
Alternatively, you could install the game in the admin account, leaving the user account set as a limited account. But this would be inconvenient in the long run.
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