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.