JSI Tip 6359. Install SP3 (or greater) or the specified hotfixes on a Windows 2000 domain controller before you run the Windows Server 2003 Adprep/Forestprep Command?

Microsoft Knowledge Base Article 331161 contains the following summary:

Before you run the adprep /forestprep command, Microsoft recommends that you install certain hotfixes and service packs on any Microsoft Windows 2000-based domain controllers. The hotfixes and service packs are listed in this article.