Reliable way of generating unique hardware ID

Question: I have to come up with unique ID for each networked client, such that:

• it (ID) should persist once client software is installed on target computer, and should continue to persist if software is re-installed on same computer and same OS installment,
• it should not change if hardware configuration is modified in most ways (except changing the motherboard)
• When hard drive with client software installed is cloned to another computer with identical hardware configuration (or, as similar as possible), client software should be aware of that change.

A little bit of explanation and some back-story:

This question is basically age old question that also touches the topic of software copy-protection, as some of the mechanisms used in that area are mentioned here. I should be clear at this point that I'm not looking for a copy-protection scheme. Please, read on. :)

I'm working on a client-server software that is supposed to work in a local network. One of the problems I have to solve is to identify each unique client in the network (not so much of a problem), so that I can apply certain attributes to every specific client, retain and enforce those attributes during the deployment lifetime of a specific client.

While I was looking for a solution, I was aware of the following:

• Windows activation system uses some kind of heavy fingerprinting mechanism that is extremely sensitive to hardware modifications,
• Disk imaging software copies along all Volume IDs (tied to each partition when formatted), and custom, uniquely generated IDs during installation process, during first run, or in any other way, that is strictly software in its nature, and stored in registry or on hard drive, so it's very easy to confuse two.

The obvious choice for this kind of problem would be to find out BIOS identifiers (not 100% sure if this is unique through identical motherboard models, though), as that's the only thing I can rely on that isn't duplicated, transferred by cloning, and that can't be changed (at least not by using some user-space program). Everything else fails as either being not reliable (MAC cloning, anyone?), or too demanding (in terms that it's too sensitive to configuration changes).

Sub-question that I'd like to ask is, am I doing it correctly, architecture-wise? Perhaps there is a better tool for the task that I have to accomplish...

Another approach I had in mind is something similar to a handshake mechanism, where a server maintains an internal lookup table of connected client IDs (which can be even completely software-based and non-unique at any given moment), and tells the client to come up with a different ID during handshake, if a duplicate ID is provided upon connection. That approach, unfortunately, doesn't play nicely with one of the requirements to tie attributes to specific client during lifetime.

It seems to me that you should construct the unique ID corresponding to your requirements. This ID can be constructed as a hash (like MD5, SHA1 or SHA512) from the information which is important for you (some information about software and hardware component).

You can make your solution more secure if you sign such hash with your private key and your software verify during the starting, that the key (signed hash value) is signed (only public key must be installed together with your software). One can expand such kind of solution with different online services, but corporate clients could find online services not so nice.