New Windows 7 ownership of old backups

Hi,

I set up a new Windows 7 on my laptop and still have backups of the old installation. Unfortunately, I changed my user name and now I don't have access to my old files if I mount the old backup with my new username. I guess, I could gain ownership in the "write mode" but then I would double the size of the backup file which is not very pleasant... How can I access my old files without doubling backup-file sizes?

Thanks!