no need for xp_cmdshell or any other nasties. Create a sql server job step as activex script and set to VBS. The code i use is as follows

Option Explicit

'Delete all SQL Server backup files more than 5 days old

Dim oFS, oSQLBackupFol, oFol, oFil

Dim sPattern

sPattern = "*.bak"

Set oFS = CreateObject("Scripting.FileSystemObject")

Set oSQLBackupFol = oFS.GetFolder("drive:\path") 'Change this as appropriate

For Each oFol IN oSQLBackupFol.SubFolders 'get subfolders of the above path

For Each oFil in oFol.Files 'get each file in subfolder

If oFil.DateCreated < Now-5 then 'Change this as appropriate

If ucase(right(oFil.name, 4)) = ".BAK" then

oFil.Delete

End If

End If

Next

Next

Set oFS = nothing

Set oSQLBackupFol = nothing