Here is a quick but hopefully useful guide for preinstalling the Virtual Server R2 Shunt Driver into Windows Server 2003.

Extract the contents from the SCSI Shunt Driver.vfd file (found in C:\Program Files\Microsoft Virtual Server\Virtual Machine Additions) to a folder, C:\Drivers\MSVMSCSI in this example (Note: move the content in Win2003 folder up one level, eg. the same level as txtsetup.oem)

The content of the C:\Drivers\MSVMSCSI folder should have the following files

MSVMSCSI
msvmscsi.inf
msvmscsi.sys
TXTSETUP.OEM
vmadd_msvmscsi_sys.cat

Then modify the TXTSETUP.OEM file to look like below

[Disks]
d1 = "Microsoft VM Accelerated Additions Driver Disk (Windows Server 2003)", \msvmscsi, \

[Defaults]
scsi = MSVMSCSI_2003

[scsi]
MSVMSCSI_2003 = "(Windows Server 2003) Additions Accelerated SCSI Driver", msvmscsi

[Files.scsi.MSVMSCSI_2003 ]
driver = d1, msvmscsi.sys, msvmscsi
inf    = d1, msvmscsi.inf
catalog = d1, vmadd_msvmscsi_sys.cat

[HardwareIds.scsi.MSVMSCSI_2003]
id = "PCI\VEN_9004&DEV_7078&SUBSYS_70781414&REV_02","msvmscsi"

On the distribution share, containing the Windows Server 2003 SP1 setup files, create a folder structure like \i386\$OEM$\TEXTMODE, copy the C:\Drivers\MSVMSCSI content to this folder.

On the distribution share, containing the Windows XP SP2 setup files , create a folder structure like \i386\$OEM$\$1\Drivers\MSVMSCSI, copy the C:\Drivers\MSVMSCSI content to this folder.

Then Modify the unattend.txt file to look like this

[MassStorageDrivers]
    "(Windows Server 2003) Additions Accelerated SCSI Driver" = OEM
    "IDE CD-ROM (ATAPI 1.2)/PCI IDE Controller" = "Retail"

[OEMBootFiles]
    msvmscsi.inf
    msvmscsi.sys
    TXTSETUP.OEM
    vmadd_msvmscsi_sys.cat
    MSVMSCSI

[Unattended]
    OemPreinstall = Yes
    OemPnPDriversPath = "Drivers\MSVMSCSI”
    DriverSigningPolicy = Ignore