btw, i have tried WMP alone(no codec) and guess what, MKV cant be played...so me and my anime is safe...
but the thing is, if an anime, let say conan, is in mkv, i have to convert them to avi or mp4...
and thanks to this > And if that's not the case, what's wrong with just using Windows Media Player?
What version of Windows do you have? And I don't mean just XP/Vista/7, but also Home Premium/Professional/Ultimate/etc. If you have, I think it's Professional or above, you can use local security policies to block users from executing specific programs - thus allowing you to leave MPC or whatever installed so you can open mkv files, but block your brother from accessing it.
Control Panel > Administrative Tools > Local Security Policy. Right Click on Software Restriction Policies on the left and add one. Then Right click on the Additional Rules subitem that appears, choose New Hash Rule, point it to the program you want to block (eg. MPC), make sure it's set to disallowed, and click OK. Presto, the application is blocked.
Only thing is, that might block everyone from running it, including you. Under Software Restriction Policies > Enforcement, you can set it to not apply to administrators (so if you're an admin, but your brother's not, he'll be blocked but you won't), but I don't think you can get much more fine-grained control than that easily.
There's probably a more elegant solution for applying rules to specific users or groups, but I'm no expert on systems administration, so I'm not sure what it would be.
EDIT: If you have Windows 7 Enterprise or Ultimate, you can use Applocker to block a specific user from executing a specific application. Local Security Policy > Application Control > Applocker.