System Restore not allowing me to play my games?

I bought the parts for my machine about 3-5 months ago, and had it put together by my step dad, but the cpu was broken so we bought a core duo 2 e6300 and had that fitted in and its been a great machine. However, i've got a few snags that I could use some help with.

Firstly I have a HD with 150 or 140, I forget but when I started out my machine is started 40 gigs later, and so i've almost finished the whole HD with about 7 games and just tunes etc, not much. My older machine which I had for about 3-4 years lasted longer and it had same amount of games and tons of junk on it....and it had less gigs :(

So i'm wondering does anyone know what might have happened to the loss of 30-40 gigs, which I have no idea could've gone.

 Secondly, I had to system restore a couple times due to a couple problems occuring. And now, a couple of my games (namely Battlefield 2142) doesn't work, nor Live for speed 2(which are both mainly online). So I tried to unistall bf2142 and reinstall but it wouldn't let me get rid of it, it just does nothing. So I thought i'm going to have delete all the files and such and I have, but it still says I have 11 mb somewhere on the computer and its preventing me from playing again. So now i'm bummed :(

 So if anyone could help with a solution i'd appreciate it greatly. And also, is it possible to wipe the whole HD clean(I know it is) but how exactly would I go about that?

Thanks alot.

Thanks alot.

 If you didn't understand my other problem i'll explain little bit better.

When I system restored, which as you probably know, restores the computer to an earlier point in its life. So when i've done this, games that worked before, no longer work, and I don't particularly know why. It happens alot when people use system restore, and sometimes it may not happen. Also, when I system restored, the computer no longer allows me to uninstall the games I can't play so I have to resort to manually deleting the files associated with games, and this will allow me to install it again, because it thinks the game has been "uninstalled", but one of the games (namely bf2142) still says theres 11mb left, so I can't even install it again because of this 11mb.

I've even searched the whole of my c-drive to find where the 11mb worth files are but they don't even show up. So I have no idea what to do :(