SharedDocs ~ Time Machine

Hello.

I'm trying to use Time Machine on a Windows share (I think?). Every time I choose "backup now" I get the "The backup disk image could not be created" error.

Here is what it shows in Console:

4/15/08 6:12:21 AM /System/Library/CoreServices/backupd[304] Backup requested by user
4/15/08 6:12:21 AM /System/Library/CoreServices/backupd[304] Starting standard backup
4/15/08 6:12:21 AM /System/Library/CoreServices/backupd[304] Creating disk image /Volumes/SharedDocs/Pluto_0016cbae9acd.sparsebundle
4/15/08 6:12:49 AM /System/Library/CoreServices/backupd[304] Error 45 creating backup disk image
4/15/08 6:12:49 AM /System/Library/CoreServices/backupd[304] Failed to create disk image
4/15/08 6:12:49 AM /System/Library/CoreServices/backupd[304] Backup failed with error: 20

Can anyone help? I'd really like to backup to my network instead of my Mac mini because it has a 250GB HD.

Thanks,
Jo