You should try to avoid running Exchange on a domain controller. See: http://articles.techrepublic.com.com/5100-10878_11-6070680.html
And Microsoft says: After you install Exchange Server 2003 on a server, do not change the role of the server. For example, if you install Exchange Server 2003 on a member server, do not use the Dcpromo tool to promote the server to a domain controller.
So I would not take that step. It seems like it would be more adviseable to replace the bad hard drive. Next time (especially at a co-lo) you should setup your RAID array to include a hot-spare.