If you install Windows 7 or Vista by performing a custom installation and don't format the partition during the installation process, files that were used in your previous version of Windows are stored in the Windows.old folder.
User folder under windows.old will contain pictures, files and documents of all the users.
Please take a back up of all data and folders onto a external hard drive before deleting this folder.
1. Open Disk Cleanup by clicking the Start button. In the search box, type Disk Cleanup, and then, in the list of results, click Disk Cleanup.
If you're prompted to choose a drive, click the drive you just installed Windows on, and then click OK.
2. In the Disk Cleanup dialog box, on the Disk Cleanup tab, click Clean up system files.
If you're again prompted to choose a drive, click the drive you just installed Windows on, and then click OK.
3. Select the Previous Windows installation(s) check box, and any other check boxes for the files you want to delete, and then click OK.
4. In the message that appears, click Delete Files.