You could use your hard drives serial number as a unique ID as you probably won't be removing it or you could use your MAC Address from the installed network interface card (NIC) however i would recommend using ther hard drive serial number as you can have more than one NIC and it is more likely to be changed than your hard drive

Computers can also have multiple hard drives. And I've changed hard drives more frequently than NICs. Possibly there is some unique ID in the bios that can be retrieved, I don't know.

Yes, that is true, myself I have 4 harddrives actually but the idéa was interesting anyway.. I will start search for methods and see what I can come up with.

To be honest you're right I think what I meant is get the ID of some piece of hardware you don't intend on changing and that can be the identifier.
You could also generate some unique ID and keep it in a text file. Then check the location you put it for the unique ID.

>>I am searching for a way to identify a computer so a software can recognice that computer in order to work.

There really is no idiot-proof way to prevent someone from doing that. Its just like encryption code -- it only works for the novice. A good hacker could probably break any method you try to use.

So, over a period of a couple of years, I do
- upgrade the RAM
- replace the hard disk
- replace the graphics card
- replace the NIC
- replace the motherboard and processor.

Do I still have the same machine?

Overly elaborate "machine locked" software only really serves to piss off genuine customers who paid for it, only to be met with abuse from the s/w vendor when they've "upgraded" the wrong thing.
The last thing you need is paying customers taking their business somewhere else!

People who pirate your s/w were going to pirate it anyway. Any s/w which is vaguely useful will be cracked (and with the internet, can be found by anyone willing to look).

You might be better served by spending your time on improving the features, and removing the bugs, so that more genuine people would want to buy it.
As opposed to spending time and money on people who were never going to give you any money to begin with.

"Nice security system, shame about the program" is all to common.

Or change your business model so you give the s/w away and you sell service to those that want it. The program in effect becomes an interactive advert for what you're really selling, and which is a lot harder to rip-off.

Yes that is true, I really understand the problem with this, as Ancient said a good hacker have thought about ´all´ the possible ways.
I might just choose a way and wait for it to be cracked
As I have seen you can almost search for any software out there and find cracked versions.

We can just hope for some honest people who like the feeling to buy it as it should be. I wonder what the statistic is for that

You could get the computer's hard disk ID (hardware key), you can find some source code on how to do this here, but as already said the user can always replace a 'dead' part in his computer, so dealing with a unique computer ID will be way more difficult than it looks like on the first sight

I'm still mystified why people even bother to protect most software. When its lifespan/popularity will probably be to short to ever see a revenue drop, due to piracy.

Quick few interrupts, yes. But I suffer hard-drive failure, all the time.