boot it and see if you get "press any key to boot from cdrom" message.
press Enter and off you go.
When you get the screen asking where you want to install - click Drive Options (Advanced) - I have no idea what that is in Greek.
Click a partition and select delete, or Format.
Do the same on the other partition.
If you deleted them both - you have one large amount of space - you need to click Create and select the size you want. Then Format the partition you just created.
Make sure the partition you want to install on is highlighted - then click Next. Clean Install Windows 7