Unlicensed version MS Office to licensed – Best method?

I have a client that wants a licensed version of MS Office 2003 on his computer and is asking that I do the install, saving all his emails, docs, etc. Right now he’s running WinXP Pro and has a pirated version of Office 2000. What would be the best way to get him legal (and least expensive)? Should I have him purchase a full version, or will MS sell him a key to make his legal and then get the 2003 upgrade?