Unable to delete dated files in SQL Server maintainance plan

We had a weird problem these days. There are some backup plans setup
in our SQL Server 2000 databases, the backup files older than one week
should be deleted automatically according to the configuration of the
plan. However, it didn't work since about a month ago.

I found a similar issue proposed by marald at
http://www.winnetmag.com/sqlserver/forums/messageview.cfm?catid=1664&threadid=122719,
as below,

We have experienced this problem in the past.

Our problem was caused by our tape backup system having a handle open
to the sql backup file, and therefore preventing the maintanence plan
from being able to delete the file.

To check this, either check you tape backup logs for the time the tape
backup started and ended streaming the database backup file, or -

As mentioned by a previous post, go to www.sysinternals.com and
download "Handle v2.2". Then add a step into the job as such
(obviously changing the paths accordingly) -

handle.exe >c:\temp\handle_output.txt

This will then dump ALL handles on the system to handle_output.txt at
the point the mainanence step completes (or starts depending on where
you put it). If you search down this file for the name of the backup
file trying to be deleted, you should be able to identify the process
that has it open.

And as mentioned by Ernest Edwards
"...culprit is an on-demand virus scanner, the MS Indexing service or
other ancillary product. "