I wrote this basic VBScript sample to delete old .bak files from a particular folder if its older than X number of days.
'Remove old backups
iDaysOld = 3
strExtension = ".bak"
sDirectoryPath = "D:\MSSQL\Backups\"
Set oFSO = CreateObject("Scripting.FileSystemObject")
Set oFolder = oFSO.GetFolder(sDirectoryPath)
Set oFileCollection = oFolder.Files
'Walk through each file in this folder collection.
For each oFile in oFileCollection
If oFile.DateLastModified < (Date() - iDaysOld) Then
If Right(UCase(oFile.Name), Len(strExtension)) = UCase(strExtension) Then
Set oFSO = Nothing
Set oFolder = Nothing
Set oFileCollection = Nothing
Set oFile = Nothing