When you install windows 7 (for that matter Windows NT onwards) and create usernames, each username will be associated with a SID (Security Identifier) and internally these SIDs are used by the system and registry. This will allow you to change the username for ex. from Thomas to Tom and still work correctly.
These SIDs are unique for each username. For example you create a username called TOM, a folder named TOM shall be created in Users folder and this user TOM will have complete access for this folder and subfolders. If you delete the username TOM and recreate TOM again and logged in to the system as TOM, you will be not allowed to access the existing TOM folder in Users folder as the internal SID is different for the new user.
Similarly when you reinstall the windows 7 and create a user with same name as old user, This user can not access the old user folder.
One way you can access this folder is to take ownership of this folder and subfolders and files. Another way is to add the new user to the permissions of old user folder and subfolders and files by logging in as an administrator.