My brother installed Windows 7 on my computer. But he did not create a backup of my user data (mydoc / desktop / mypics/ downloads) on the system drive. Now is there any option to recover those lost files?
Slap your brother upside the head, he just deleted your life.
If the installation was a clean one you might be able to recover some files...give Recuva a go:
Could you please specify how Windows 7 was installed? Was it an upgrade or a clean installation? Was the system partition formatted before the installation?