My friend reformatted my entire PC, but he failed to mention that he deleted the DVD decoder for my DVD player !

Considering he
reformatted your entire PC, you would assume that everything on the PC was deleted...

God knows why he would need to do that (probably so i need to go back to him and pay him again to get a DVD decoder)

If he's a friend of yours, why would he do that?
It's more likely you didn't give explicit instructions of what to back up and what to not...
freeware / shareware
I would quote that huge WTF emoticon for great justice but it's rather big and clutters the thread so I'll just reference it. Hell, this whole community is about freeware, if you can call open source stuff that. Even discussions about piratism are not a problem, but links to pirated stuff are a no-no...
I cant watch my animes anymore !!! 
So what is the problem then? Wrong area codes or some other stuff?
Copy prevention methods such as CSS shouldn't in itself be any problem to Windows PC's they should be able to read those disks out of the box (theoretically anyway). Obviously, anyone who has Linux and wants to view most DVD's has to get a handy package called libdvdcss2 which enables the computer to read CSS-protected DVD's without any hindrances. But since your computer has had to be reformatted I would think it doesn't have a Linux installed...

If you're talking about software that circumvents the DVD area code stuff, I don't really know if that's even possible since the area codes are set on the DVD-reader firmware as far as I know.

If the problem is not having a DVD-player, I'd have to say that Media Player Classic, or VLC Player should do fine. Get both and there's not much that you can't playback.
...at any way, it's usually better to learn to do your computer-related stuff yourself, that way you know what's going on and what your PC has eaten.
