Backup in Windows XP Pro

I have two hard drives in my Pentium 4 2.8GHZ computer, and it is running under Windows XP Pro Service Pack 2. I setup backup to do a full backup of drive C: and save it on my 2nd hard drive. Then on the daily basis perform the backup incremently. By the way I am using Backup program provided by Windows XP Pro.

The backup drive begin to get full (running out of disk space). How can I delete some of the increment backup files on my backup hard drive?