ok - here's the end of the file that contains the error:
==============
16.172: System Restore Point set.
16.218: PFE2: Not avoiding Per File Exceptions.
16.218: GetCatVersion: Failed to retrieve version information from
C:\WINDOWS\system32\CatRoot\{F750E6C3-38EE-11D1-85E5-00C04FC295EE}\Tmp.0.KB893803v2_wxp.cat with error 0x57
17.047: Copied file: C:\WINDOWS\system32\msi.dll
17.125: Copied file: C:\WINDOWS\system32\msiexec.exe
17.375: Copied file: C:\WINDOWS\system32\msihnd.dll
17.578: Copied file: C:\WINDOWS\system32\msimsg.dll
17.609: Copied file: C:\WINDOWS\system32\msisip.dll
17.687: Copied file: C:\WINDOWS\system32\DllCache\msi.dll
17.718: Copied file: C:\WINDOWS\system32\DllCache\msiexec.exe
17.765: Copied file: C:\WINDOWS\system32\DllCache\msihnd.dll
17.812: Copied file: C:\WINDOWS\system32\DllCache\msimsg.dll
17.828: Copied file: C:\WINDOWS\system32\DllCache\msisip.dll
17.859: DoInstallation: Installing assemblies with source root path:
f:\e4f358d311c37f4a80e8e86e05\
17.859: Num Ticks for Copying files : 3234
17.922: DoRegistryUpdates:UpdSpInstallFromInfSection Failed for
MSI.Reg.Install: 0x5
17.922: DoInstallation

oRegistryUpdates failed
17.953: Access is denied.
23.593: Message displayed to the user: Access is denied.
==================
"MowGreen [MVP]" wrote:
> 0x80070643 - ERROR_INSTALL_FAILURE
>
> That really helps, eh Jeff ? 
> Go here :
> http://www.microsoft.com/downloads/d...displaylang=en
> Click the Continue button next to " Validation Required "
>
> After Validating, scroll down to the " Files in This Download " heading
> and download WindowsInstaller-KB893803-v2-x86.exe
> Save it, do NOT run it from it's location
> Close IE and all other open programs and apps.
> Now try to install it.
> If it fails again there will be a KB893803.log located in the WINDOWS
> directory that you can check for error codes. The latest install logging
> will be located at the *bottom* of the log.
> Post back with the section that contains said error code(s)
>
>
> MowGreen [MVP 2003-2006]
> ===============
> *-343-* FDNY
> Never Forgotten
> ===============
>
>
> Jeff wrote:
>
> > Hey folks - getting desperate here.
> >
> > I upgraded one of my boxes from XP Home to XP Pro last week and since doing
> > that I have been unable to do automatic updates. When trying to install
> > Windows Installer 3.1 it downloads the install and then chokes part way into
> > it with an Error Code: 0x80070643
> >
> > I've crawled around out here an on the broader web and tried many, many
> > possible solutions none of which work. The highlights:
> >
> > I do not have any office products installed on this box - not related
> > I have not run any registry cleaning utilities
> > I downloaded the WGA Diagnostic tool and it gives my system a thumbs up.
> > Tried the "Use administrator options" trick from another thread out here -
> > no go.
> > gone through every single related option in the Windows Update
> > Troubleshooter (http://v4.windowsupdate.microsoft.com/troubleshoot/) page -
> > nothing helped.
> > There is no virus scanning or any other 3rd party software on the box - just
> > MS firewall enabled.
> >
> > I sent a note to Microsoft themselves but am still waiting to hear back -
> > Anyone got any other suggestions at to what to try? Still kicking myself for
> > upgrading from Home just to get remote backups working with True Image...
> > stupid, stupid, stupid...
> >
> > Any and all suggestions greatly appreciated - really want to make sure my
> > box is updated since leave it on 24/7 most of the time (though not now).
> >
> > thanks
> >
>