In order to succesfully convert Windows systems to a VMware virtual machine, you need to install the sysprep files of the to be converted host. Below is a list of downloads to these sysprep files:

The base folder of where to put the files is always:

Quote

%ALLUSERSPROFILE%\Application Data\VMware\VMware vCenter Converter Standalone\sysprep

Windows 2000 (sysprep 1.1)

• Extract contents to: base-folder1.1
• Link: http://www.microsoft.com/downloads/details.aspx?FamilyID=0C4BFB06-2824-4D2B-ABC1-0E2223133AFB.

Windows 2000 SP4

• Extract contents to: base-folder\2k
• Link: http://download.microsoft.com/download/D/2/C/D2CBC4DA-5F0D-435F-AE2C-FE9FCB6BDA8E/sp4DeployTools.exe.

Windows 2003 SP2 (32-bit)

• Extract contents to: base-folder\svr2003
• Link: http://www.microsoft.com/downloads/details.aspx?FamilyID=93f20bb1-97aa-4356-8b43-9584b7e72556.

Windows 2003 SP2 x64

• Extract contents to: base-folder\svr2003-64
• Link: http://www.microsoft.com/downloads/details.aspx?familyid=C2684C95-6864-4091-BC9A-52AEC5491AF7.

Windows 2003 SP1 (32-bit)

• Extract contents to: base-folder\svr2003
• Link: http://www.microsoft.com/downloads/details.aspx?familyid=A34EDCF2-EBFD-4F99-BBC4-E93154C332D6.

Windows XP SP2 & SP3(32-bit)

• Extract contents of CAB to: base-folder\xp
• SP2 link: http://www.microsoft.com/downloads/details.aspx?familyid=3E90DC91-AC56-4665-949B-BEDA3080E0F6.
• SP3 link: http://www.microsoft.com/downloads/details.aspx?FamilyID=673a1019-8e3e-4be0-ac31-70dd21b5afa7.

Windows XP SP2 x64

• Extract contents to: base-folder\xp-64
• Link: http://www.microsoft.com/downloads/details.aspx?familyid=C2684C95-6864-4091-BC9A-52AEC5491AF7.
  Note that this link is the same for 2003 x64.

For SBS, use their standard counterparts.

Source:

• The VMware vCenter Converter Standalone help
• http://vm-help.com/esx/other/sysprep_download_links.html

Notice

Starting from Windows Vista, the sysprep tools have been built in.

EDIT: I've added several deploy.cab files because the installables do not allow easy access to the files unless you actually install it on a Windows matching the package's OS.
It's a bit sad that Microsoft does not re-release these packages as simple CAB-downloads so they can be used with VMware for P2V-ing machines.