It depends on what exacty you configured Windows Backup to back up, and what your drive/partition setup looks like.
Windows Backup can create two types of backups:
- For user files, it compresses the files into a bunch of ZIP archives (apparently trying to keep the size of each ZIP file below 200 MB). These can be opened with any decompression tool that supports the ZIP format (which is probably all of them). Although it's a bit cumbersome to search through them for individual files, it should be possible to mass extract them all into a folder of your choice.
- For the system partition (which, if you have just one partition, might also contain your data), it creates a virtual hard disk (VHD) file. These can be mounted in Linux, and the contents browsed just like a regular hard drive.
So either way your backup should be accessible under Ubuntu. It's not quite as easy as clicking "Restore", and you might find @ChrisF's advice to use an external hard drive to be more convenient, but your data is definitely not locked away for good.