As one that has been assigned this type of task before, several items including what "valis" had to say are necessary to carrying it out. Hopefully, you have access to some type of listing that contains all serial numbers and date of purchase of each and every machine in your organization. With this list, you can have a very strong beginning to the end of your project. Simply run an age list against this list and single out all machines that meet the age requirement as set by policy of you organization. Then the time consuming part takes place if you don't have an active inventory and that is to physically perform an inventory that this will be used from this point forward for future PC rollouts marked to replace machines that are aged enough to meet policies. You can probably ascertain what you need to do after this inventory is completed. You can be a hero in doing this sort of assigned project or very easily end up in the unemployment lines with a very slight skew in your record keeping....I'm sure you understand. Anyway, good luck with this...depending on the size of your organization, you'll need it!
CMS