You don't always need to create a computer account first. In fact, I almost never do that. Once I changed the workstation to join the domain, it required a domain admin username/password. After supplying that it created the computer account for me. I'm assuming you did a similar thing. You are correct in that this account will have a different SID than the previous.
 
Sometimes you will run into problems if the workstation name is exactly the same as the previous, and it has not been removed from the DC, however there are workarounds for that.
 
-RY

In an oversimplified explanation, it basically took your word (as an admin) that you wanted to overwrite the existing object name with the new SID from the new install. This works 99.9% of the time for me, and I only had a couple of odd issues when a server came offline and I couldn't get the DCs to play nice and update their SAMs (NT4 domain a while back). Now, if you were to be dual booting and installed one instance of W2K in a domain with a particular name, and then created ANOTHER installation on that same PC and used the same name, the second installation would work properly while the first one would be denied access to the domain. The domain would recognize that it was old configuration, but would consider it invalid (which is what you want, unless you are stuck and that that badboy to come back which is a whole other story ).

WEll, when i have reloaded WIn2k in the workstation, i have keep the same name and never had problem login to my domain. ITs what i dont understand...