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You can add a server 2003 DC just fine as long as the domain functional level is 2003 or lower. The default install mode for Server 2008 is to be 2003-compatible.<br />
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As dariusg stated, you need to make sure that your 2003 server's DNS settings point to the 2008 server, assuming it is also a DNS server.
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I setup a windows Server 2008 DC . Active D is working Fine. In short everything seems to be working. except I can't add a Server to my domain i get the following error&quot; <br />
an attempt to resolve the dns name of a dc in the domain being joined has failed.Please verify this client is configured to reach a DNS server that can resolve DNS name in the target domain &quot;<br />
I can add a workstation fine I can do &nbsp;\\dc name it works fine I can access share ok.<br />
&nbsp;I can see all my Computers in my domain ok<br />
I can't only add additional server 2003 or 2008 to the domain. 
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I setup a windows Server 2008 DC . Active D is working Fine. In short everything seems to be working. except I can't add a Server to my domain i get the following error" 
an attempt to resolve the dns name of a dc in the domain being joined has failed.Please verify this client is configured to reach a DNS server that can resolve DNS name in the target domain "
I can add a workstation fine I can do  \\dc name it works fine I can access share ok.
 I can see all my Computers in my domain ok
I can't only add additional server 2003 or 2008 to the domain. 

Adding additional Server 2003 to  Windows 2008 Domain Controller

Network security consists of the policies adopted to prevent and monitor authorized access, misuse, modification, or denial of a computer network and network-accessible resources. Network security involves the authorization of access to data in a network, and covers a variety of computer networks; conducting transactions and communications among businesses, government agencies and individuals. Networks can be private, such as within a company, and others which might be open to public access.

Network Security

Windows Server 2003 was based on Windows XP and was released in four editions: Web, Standard, Enterprise and Datacenter. It also had derivative versions for clusters, storage and Microsoft’s Small Business Server. Important upgrades included integrating Internet Information Services (IIS), improvements to Active Directory (AD) and Group Policy (GP), and the migration to Automated System Recovery (ASR).

Windows Server 2003

Windows Server 2008 and Windows Server 2008 R2, based on the Microsoft Vista codebase, is the last 32-bit server operating system released by Microsoft. It has a number of versions, including including Foundation, Standard, Enterprise, Datacenter, Web, HPC Server, Itanium and Storage; new features included server core installation and Hyper-V.