When doing technical support. People don't always have their reinstall CD, that seems reasonable... So I get an ISO of their version and edition from off of MSDN.
What files do I need to copy over from their existing installation so that Windows won't bother with reactivation?
This is not piracy, just a common technical support issue. I don't want to resort to using those pirate tools available online. I want to know how this is done cleanly.
Saying, Microsoft doesn't allow this makes me look like an idot in front of the customer. They have a legitimate complaint, how are they supposed to know they must keep the reinstall disk?
UPDATE: When the hardware changes, the OS will still deactivate like normal. They should be allowed to reinstall a copy of their OS. Not being allowed to has to be a violation of some civil code or other law...
If it's a legitimate installation, you can call Microsoft and explain to them why it needs to be activated. Activation information is stashed in the registry, as far as I know, and the registry is pieced together from other files. Unless you had precise keys to copy you'll end up with possibly corrupt data from other applications. In my experience it's far more hassle to salvage than to nuke it from orbit.
Call MS. Or contact their vendor. The vendor can supply reinstall disks in most cases for their customers on request. Or look to see if there's a restore partition.
It's not your fault or anyone but the customer's fault if they didn't keep their install media or documentation. It often comes on CD's that are marked as restore CD's or OS CD's, or in envelopes that have dire warnings to not lose them. This is as much the customer's fault as it is that they didn't keep backups. And I know average home users don't do it. Some life lessons suck to learn.
You're asking for how to do something without really mentioning WHY you want to do it (only what's happened). I'm going to assume that by saying they've lost their CD, what you actually mean is that they've lost their product key?
If so, Magic Jellybean is your friend. It won't stop you from having to re-activate, but it will tell you what the product key is. Even if the machine is fried, if you have a copy of the registry (i.e. the machine won't boot but you can put the HDD in another computer) you can extract the Windows key from there.
And if you have the original product key, on the original hardware, there shouldn't be any issues with re-activating.
The only flip side to this is if the original product key is OEM (e.g. it's a Dell product key, and it came pre-installed on a Dell PC) then this approach is useless as those OEM keys won't be accepted by anything except the Dell VLK image (which you can't get from MSDN).
If these are customers that you've dealt with for a while - particularly if you helped them set up their PC in the first place - you need to tell them to keep all their original disks. You should probably help package them up for the customer.
If you didn't help set up their system, then your best bet is to contact the vendor and see about getting a copy of the install CD. I don't know about other vendors, HP is good about this, I'd imagine the other big names are as well. If not from the vendor, maybe someone else in the same office has the same machine and you can borrow a CD from them. Or, if it's a white box OEM copy, you might be able to get another generic OEM disk to use.
Then, as Mark said, the license key should be on the PC, so you can install from there.
Other than that, if they didn't keep the disk and don't have a recovery partition, then they're stuck - they need to buy a new copy. I don't see what the problem is explaining this to a customer, especially if you didn't help them set up the machine in the first place. It's simply how MS does it. Yes, it can be confusing to them when they have a bunch of CDs with their new machine, but if they don't keep the ones that say "Important" on them, there's nothing you can do after the fact.