mdf files too big in SQL Server 2005

I hope someone can advise me on what to do here.

I am running my database on SQL Server 2005. I had created a backup maintenance job myself using the SQL Server Management Studio, when using a maintenance plan I set up this to delete files based on the age of the file at task run time older then 1 week.

However some one else wanted to use MOM and do the back up there and this I think is appending the backup each day which now leads to the back up for this one database being 4,350,784 KB and this is causing problems with my database load.

I want to either be able to set up something in MOM for the database backup to delete anything over 1 week old or stop this backup altogether and go back to using Management Studio.

Can anyone advise me on what is the best to do and what to do if I do go back to using Management Studio as how do I can decrease the size of the .mdf file as I have already tried to shrink and now this is also hanging on me??

Please help.

Thanks