This will overtly depend on how you upgraded your system. If you upgrade from Windows Vista they would be in the same location in which you had stored them. For a custom upgrade from XP without reformatting the old files will be stored in a folder called Windows old. To RETRIVE the files follow the steps below
1: Open the DRIVE where you have installed Windows (usually, the C: drive)
2: Double-click the Windows.old folder
3: Double-click the Users folder
4: Double-click your USER name
5: Open the folders that contain the files you want to retrieve. For instance, to retrieve files in the Documents folder, double-click Documents
6: Copy and paste the files that you want from each folder to a folder in Windows 7
For each user ACCOUNT repeat the steps 4 and 6
In case you reformatted your drive or partition the files cannot be recovered.

This will overtly depend on how you upgraded your system. If you upgrade from Windows Vista they would be in the same location in which you had stored them. For a custom upgrade from XP without reformatting the old files will be stored in a folder called Windows old. To retrive the files follow the steps below
1: Open the drive where you have installed Windows (usually, the C: drive)
2: Double-click the Windows.old folder
3: Double-click the Users folder
4: Double-click your user name
5: Open the folders that contain the files you want to retrieve. For instance, to retrieve files in the Documents folder, double-click Documents
6: Copy and paste the files that you want from each folder to a folder in Windows 7
For each user account repeat the steps 4 and 6
In case you reformatted your drive or partition the files cannot be recovered.