I had this issue recently when trying to update exhcange 2003 prior to a migration to exchange 2010. Setup failed while installing sub-component Exchange ActiveSync with error code 0xC0070643. This is down to an old file being overwrote during service packs so when it comes to patch the file, it does not exist anymore.
The actual error is down to an MSXML3.MSI conflict.
- Get a copy of the latest MSXML3 from herehttp://www.microsoft.com/downloads/details.aspx?FamilyID=28494391-052b-42ff-9674-f752bdca9582
- Place a copy inside Exchsrvr\bin and also E3SP2ENG\setup\i386\exchange\bin of SP2 extracted folder.
- Rename the existing msxml3.msi to msxml3.msi.backup
- Make a copy of the latest version MSXML Windows Installer file and rename it to msxml3.msi. This will leave you with msxml3.msi and msxml3sp?.msi (in my case msxml3sp7.msi).
- Click Retry on the error and it should continue as planned.
Tags: service pack