The Basic System Administration guide for VirtualCenter 2.5 requires that the sysprep files be extracted to the VirtualCenter server for guest customization to be available when cloning a virtual machine. Below are the download links for various Microsoft OSes and specific extraction instructions.
Sysprep 1.1 files
Location: C:\Documents and Settings\All Users\Application Data\VMware\VMware VirtualCenter\sysprep\1.1
Download: [download]
Instructions: Extract the EXE download and copy the contents of the tools folder to the 1.1 folder.
Windows 2000
Location: C:\Documents and Settings\All Users\Application Data\VMware\VMware VirtualCenter\sysprep\2k
Download: [ SP4 download]
Instructions: Extract the EXE download and then open this file: i386\deploy.cab. Copy the contents of the file to the 2k folder.
Windows 2003
Location: C:\Documents and Settings\All Users\Application Data\VMware\VMware VirtualCenter\sysprep\svr2003
Download: [SP2 download]
Instructions: Run the EXE and then open this file: c:\windows\system32\deploy.cab. Copy the contents to the svr2003 folder.
Windows 2003 x64
Location: C:\Documents and Settings\All Users\Application Data\VMware\VMware VirtualCenter\sysprep\svr2003-64
Download: [SP2 download]
Instructions: Extract the contents of the EXE and then extract the file SP2QFE\deploy.cab. Copy those files to the svr2003-64 folder.
Windows XP
Location: C:\Documents and Settings\All Users\Application Data\VMware\VMware VirtualCenter\sysprep\xp
Download: [SP2 download]
Instructions: Open the CAB file download and copy the contents to the xp folder.
Windows XP x64
Location: C:\Documents and Settings\All Users\Application Data\VMware\VMware VirtualCenter\sysprep\xp-64
Download: [SP2 download]
Instructions: Extract the contents of the EXE and then extract the file SP2QFE\deploy.cab. Copy those files to the xp-64 folder.