There is a topic in Help and Support called "Back up and restore: frequently
asked questions" that contains instructions for deleting a backup. I'll
paste the contents below (minus formatting).
To create a new full backup, go to Backup and Restore Center and then click
the "Change Settings" link under the "Back up files" button. Then click the
"Change backup settings" option. Go through the wizard again. On the last
page, you have the option of creating a new full backup.
If your backups are saved to CDs or DVDs, you can discard discs containing
earlier backups, but make sure that you keep the most recent backup of your
files. If your backups are saved to an internal or external hard disk, you
can delete a backup by following these steps:
Open the location where the backup is saved.
For example, if you backed up your files to an external hard disk labeled
"E," connect the external hard disk to your computer, and then open drive E.
Right-click the folder containing the backup you want to delete, and then
click Delete.
Notes
Backups are saved in this format: <backup location>\<computer name>\Backup
Set <year-month-day> <time>. For example, if your computer name is Computer,
your backup location is E, and you backed up on April 2, 2006 at 16:32:00,
that backup would be located in E:\Computer\Backup Set 2006-02-04 163200.
You would right-click the folder named Backup Set 2006-02-04 163200 to
delete that backup.
When you make a full backup, a backup folder is created and labeled with the
date for that day. As you add updates, that date stays the same, but your
backup is not out of date. The next time you make a full backup, a new
backup folder is created and labeled with the date for that day, and any
updates are then added to that new folder. You should not delete the current
backup folder.
