Microsoft Office 2010

Hello all,
Hope I will able to find answer for this question.

I am in way of updating my exchange server to 2010 however I have few users who are using office xp which i want to upgrade to office 2003 for compatability for exchange 2010.

As of now I cant buy office 2003 so we decided to buy office 2010 and use this key for installation of office 2003.

My question is will office 2010 key work with office 2003 ?
If not what is work around for this as I dont want few users to be on office 2010 and all others on office 2003?

Pls help.
Thanks in Advance.

Kiran.