Claire,
Interesting post. Your solution won't work. jieyin doesn't want to RESTORE any of the data at this time. He/She wants to get rid of some of the backup data.
Jieyin,
Can't do it. It's all or nothing. Here's a possible solution:
Let's say you only want to keep one months worth of backups.
-You backup daily to mybackup.bak.
-After two weeks, you rename that file (ex. mybackupold.bak). Then you will have two backup files, mybackupold.bak and mybackup.bak will be recreated with the next backup job.
-After another two weeks, you delete mybackupold.bak and rename mybackup.bak to mybackupold.bak.
-SQLBill