I found this thread interesting on VMware Communities
have spent the last few days trying to find a free backup solution to the newly free ESXi for windows only enviroments (in particular Windows XP). The solution for me was the following:
1. Installing Windows Services for UNIX (WSFU)
2. Copying the ESXi Server password and group files to Windows
3. Configuring WSFU for accepting ESX Server connections
4. Sharing the Windows folder for NFS compatibility
5. Configuring the ESXi Server to mount the Window NFS Share as Datastore.
6. Setup Backup Script
Attached is the complete steps.
I have yet to try this, but I think this is worth looking at, and at the very least worth trying.