if you have DVD/CD for the server 2008, you can boot your computer from the DVD, then you can re-partition and format the hard drive which installed your server 2003. The better is, before you delete the server 2003, you should make sure you have a DVD copy for the server 2003. In the future, maybe you want to get back for the server 2003. And, if you have some special hardware configuration, you should record them first and download the supported driver for server 2008 for your hardware. Or you can call Microsoft service for detailed help. The Microsoft phone service is very useful.