I recently had the pleasure(?) of performing an SBS 2003 migration, going to SBS 2008. The process is very involved and I cannot stress enough that you should follow the MS white paper regarding this, To. The. Letter.
One of the first things which has to be done is to bring the source server fully up to date with all Service Packs and security updates. In fact migration cannot occur if there is not a minimum level of SP applied.
One of the SPs required is SBS Service Pack 1. This is not to be confused with Windows Server 2003 SPs which are different. SBS SP1 is a prerequisite for migration to even begin.
So I downloaded it and attempted to install it.... it chugged along for a while and failed. The error I received indicated that there was an Active Directory or possible permissions problems. Checks on AD and permissions showed no problems.
Further research indicated that it was a problem with the SBS Fax service. As it was not in use, I removed it, rebooted the server and tried to apply SBS SP1 again. It still failed. But seemed to get along a bit further. But without this installing correctly I could not install other needed updates and begin the actual migration itself.
Nothing I found on-line helped, despite many hours of searching.
After giving up on the Event log viewer, Google and Experts Exchange (which is usually great) I decided to go through the many error log files which were generated by the SP software installation step by step. The default location for these files are C:\Program Files\Microsoft Integration\Windows Small Business Server 2003\logs\
In the end I Finally solved the problem when I discovered there were several MSI files which the Service Pack software was expecting to find, that were not available in the location it expected.
A search for the MSI files were fruitless, nowhere to be found on the server. Not only that but the actual CD drive letter was different to that which was stored in the registry for installation media.
I made several registry changes to point to the locations of these install files, after I downloaded fresh versions of them from the Microsoft web site.
However this only allowed the SBS SP1 installation to progress a bit further before falling over. As I could not download all the MSI files I needed.
The thing which finally solved the problem was to change the registry key which indicated where the SBS installation CD would be found. Inserted the SBS CD #2 and run SBS SP1 again. This time it looked to the CD for those MSI files which could not be found anywhere else.
None of my on-line research indicated that this was the problem. I guess sometimes you just need to read through multi-megabyte log files...
One last thing, when you are going through the network protocol settings on the target (SBS 2008) server, whatever you do, do not turn off IPV6. You will regret it about 0.2 seconds after you perform one of the final migration tasks and demote the source (2003) server. If IPV6 is turned off, Exchange will utterly croak. Exchange 2007 and above absolutely relies upon IPV6, whether you use it on your network or not.
Hope this helps someone....