No Applications At All No SN No 'More Info'

So I got a huge problem everyone....

I just bought a computer off a guy online and I don't know what's wrong with it.. When I load up 'System Prefrences' Only 'Mouse, Keybaord and BlueTooth' Shows up on the list of what I can edit..

Also When I click on 'About this Mac' I can't click 'More Info' And there is no Serial number..

I know the computer is real though because it came with the original box and it's a very nice 20" 2008 iMac

There is nothing on this tho, like I thought about going onto the Mac App Store and it's not even on here so I can't get to it.

It's almost like I have the Mac OS.. But nothing else works... Please help as soon as possible, I've looked up everything I possibly could and couldn't find anything like this.

Also I just tried to download google Chrome and when I tried to open it I got the error message 'This can not be downloaded on this version of Mac OS'

It's running 10.5.8 but it's not the regular 10.5.8 it's like a edited version so I can't do anything... Is there anyway I can fix this. Even the display model at the apple store is more open then this.