Once you've selected a vendor to perform your deployment, you need to define very clearly the deliverables for the project. Make sure that you have clear expectations of what will be done, when it will be done, and how much it will cost, and that these expectations have been communicated to the vendor. It's customary for deliverables to be added to a project midstream, but bear in mind the addition of new deliverables to the original contract will extend delivery dates and increase the cost.
When writing your agreement with the vendor, include service level agreements that include penalties for failure to meet deadlines and failure to implement specific deliverables. Any vendor who's been successful in completing a deployment project understands the implications of these types of failures and what they could cost your business. Your vendor should have no problem agreeing to compensate your organization should any failures arise.
The lastand perhaps most importantitem you need to consider when outsourcing a Windows 2000 deployment is that, in order to be successful, you can't remove yourself completely from the process. Be prepared to work side by side with your vendor, providing guidance when necessary and taking some knowledge transfer from the vendor in the process. This will ensure that the deployment meets the expectations that you set at the beginning of the project and that you have a complete understanding of your enterprise deployment when the project is finished.