Can I manage Windows 7 machines with an open source server?
Solution 1:
I'm in a similar position, except we've got nearly 50 machines. I'd like to be able to do everything with Group Policies. This does however REQUIRE a Windows Domain Controller.
Look at the costs involved. Your time to manage the network is expensive, whereas the amount of time you could save, and be working on other projects which are more beneficial to your employer, is much more valuable. (I'm assuming here that you're not a dedicated desktop support guy)
So, a pair of domain controllers, say from HP: DL320 4GB RAM will be more than adequate. £899.00 ex VAT each.
Then Windows Server 2008 R2 - £474.00 ex VAT
Total cost is just shy of £3000
Yes, you need 2. These are going to run your network authentication, without Domain Controllers, your network will rapidly go south. It's a small cost for a massive piece of mind.
And then you'll be ready to manage your growing windows network a lot more robustly than you can with Ubuntu and Samba. I'm all for open-source, but not when it comes to managing windows networks. I've been there personally, with Slapd and Samba, and openldap schemas, and let me put it like this. It's really not worth your time.
Solution 2:
There isn't a good open source server implementation that has all the bells and whistles of AD (particularly Group Policy). Samba 4 is going to do what you'd like, but it's still in alpha.
You can fake a lot of what you want with WPKG and a slew of scripting, but you won't get the GUI management goodness of AD.
Solution 3:
http://www.zentyal.org/ but keep in mind that no open source solution can give you Group Policy. For that you'll have to pay for a Windows Server 2008 + CALs.