I recently needed this info and have been asked as well in the past and could not find a list anywhere with what is on the Base images so here is the information.

Disc 1:

Base04A.exe: Microsoft Windows Server™ 2003 Enterprise Edition

Base04B.exe: Windows Server 2003 Enterprise Edition, Microsoft Visual Studio® .NET 2003 Enterprise Architect, Microsoft SQL Server™ 2003 Enterprise Edition Service Pack 3

Base04D.exe: Windows Microsoft Small Business Server 2003 Premium Edition

Disc 2:

Base05B.exe: Microsoft Windows® XP Professional Service Pack 2, Visual Studio® .NET 2003 Enterprise Edition, SQL Server™ 2000 Developer Edition Service Pack 3

Base05C.exe: Windows XP Professional Service Pack 2, Microsoft Office Professional Edition 2003

Disc 3:

Base04C.exe: Windows® XP Professional Service Pack 1a, Office Professional Edition 2003

Base05A.exe: Windows XP Professional Service Pack 2

Base05D.exe: Windows Server® 2003 Enterprise Edition Service Pack 1

Base06A.exe: Windows XP Professional Service Pack 2, Visual Studio® 2005 Professional Edition, SQL Server™ 2005 Developer Edition

Disc 4:

Base07A.exe: Windows Vista™ Enterprise

Disc 5:

Base08A.exe: Windows Server® 2008 Enterprise Edition

Base08B.exe: Windows Server® 2008 Enterprise Edition Server Core installation

Base08C.exe: Windows Vista™ Enterprise Service Pack 1

Disc 6:

Base08D.exe: Windows Vista™ Enterprise Service Pack 1 + Microsoft® Office 2007 Professional with SP1

Read the complete post at http://owsug.ca/blogs/brad/archive/2008/11/09/Microsoft-Learning-Base-VHD-images.aspx