Reliable way of generating unique hardware ID

What you're looking for is the Windows WMI. You can get the motherboard ID (which is unique across the same type of motherboard) or many many other types of unique identifiers and come up with some clever seeded function to generate a UHID. Whoa did I just make up an acronym?

And if you're looking specifically for getting the Motherboard (BIOS) ID:

WMI class: Win32_BIOSNamespace: \Root\Cimv2

Documentation: http://msdn.microsoft.com/en-us/library/aa394077(VS.85).aspx
Sample code: http://msdn.microsoft.com/en-us/library/aa390423%28VS.85%29.aspx

Edit: You didn't specify a language (and I assumed C++), but this can be done in Java (with a COM driver), and any .NET language, as well.

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.

Many programs use the hostId in order to build a license code (like those based on FlexLM). Have a look at what Matlab does depending on the operative system:

http://www.mathworks.com/support/solutions/en/data/1-171PI/index.html

Also have a look at this question:

Getting a unique id from a unix-like system

Once I also saw some programs basing their licenses on the serial number of the hard drive, an maybe that is the less likely thing to change. Some would suggest to use the MAC of your ethernet card, but that can be reprogrammed.