Where's my space??

Hi everybody!
I bought a MacBook two days ago with Tiger,
had it served and Snow Leopard installed, went on to install
Firefox, Echofon, Adium and the iPhone SDK.
Now, after this, my 60gb harddrive shows 19gb Free.
To my count the OS should be around 10gb, roughly?
The minor programs shouldn't matter, the SDK should be, what, 2 +3?
So that's 15 (counting high 20) gig in use. But 40 is taken up....
Did I miss something here?
Where are my remaining 20gb?

Buckwheat said:

I bought a MacBook two days ago with Tiger,
had it served and Snow Leopard installed, went on to install
Firefox, Echofon, Adium and the iPhone SDK.
Now, after this, my 60gb harddrive shows 19gb Free.

Click to expand...

Do you know what it showed before?

Sadly no,
but there seems to be leftovers from the previous owner, despite new OS.
I just have no clue how to plow through what's what, but the applications folder isn't unusually big in itself, iPhoto, iWeb, itunes and garageband being the only things showing above 100mb