|MyLockBox should still be able unlock your files on H: drive even after the software has been removed and re-installed.|
And even if it can't, you can access your hidden files by booting your PC from an Ubuntu Desktop DVD. This is because your files are only hidden on Windows, not on Ubuntu.
Ubuntu takes no notice of Windows utilities like MyLockBox, and MyLockBox cannot hide your files when you are running Ubuntu.
You do not have to install Ubuntu. Just boot your PC from the Ubuntu Desktop DVD when you've made it, and you will be able to choose whether to install it or just run it from the DVD.
My advice is not to use anything like MyLockBox anyway. If you want to keep some files private, write them to CD then put the CD out of sight.