2003 is a single disk.  2003 R2 has a second disk.  If you have lost it, you will need to try and find it someplace like eBay (or a local school/college, IT shop, etc.), as Dell no longer carries replacement media for any release of Server 2003.
I just reinstalled server 2003 and now the system is asking me for disk 2. I don't remember ever having this disk and know I don't now. How can I get the second disk?
How to get new Server 2003 R2 Disk 2