Very optomistic about those records Jens. I agree that it should be way easy to do, but reality sets in and often one finds that it's quite impossible for many shops to do such an easy task and would take longer audits of there records to come up with the needed data. That said, yes doing better record keeping should be in order but doubt it will happen for most and is beyond the scope of most IT Admins.

As well, it depends on what accuracy one is looking for too. One could buy computers that are maybe a year old already from some vendors as some shops buy used and not new computers to keep costs down.

WMI is probably your best bet without doing a physical inventory.