I run win 7? how can It be so large? the DVD holds about 4GB? Does it just "reserve" the space without using it?
The Windows disk contains the files in a compressed format. One of the first things the Windows installation process does is copy the files to your disk and expand them. Then you're [hopefully] downloading updates for it, in particular Service Pack 1. My Windows directory at this time is 33.5 GB, although I've seen it as low as 20 GB in the past.
If you want to see a graphical representation of exactly what is eating up space on your SSD, you could try TreeSize. It's free and pretty cool. Just go to the Scan menu and select the disk.
(That's my own 128 GB SSD, by the way.)