Why can’t I install OS X 10.6.6 combined updates

To be clear, I can't install any Mac OS X update at the moment. I reformatted my disk and installed fresh off my Snow Leopard disk. I then continued to download 1.16GB worth of combined update (10.6.6) and I kept getting an "invalid checksum" error when attempting to open the DMG.

I'll be very clear on this to stop going around in circles. I have spent the last 2 days looking at this issue, trawling through forum posts, re-installing, verifying and checking my disk, trying 10.6.1 instead, reformatting again, doing a zero fill, the works. Anything that a forum seems to have suggested has not worked for me whatsoever.

Here's an output from my log:

Mar 1 19:37:43 Kieran-Seniors-MacBook-Pro installd[182]:
PackageKit: Install Failed: PKG: extracting "com.apple.pkg.update.os.10.6.1"
Error Domain=PKInstallErrorDomain Code=110 UserInfo=0x10011a1e0 "An error
occurred while extracting files from the package “MacOSXUpd10.6.1.pkg”."
Underlying Error=(Error Domain=NSCocoaErrorDomain Code=259 "The file couldn’t be opened because it isn’t in the correct format.")
{
NSFilePath = "/var/folders/zz/zzzivhrRnAmviuee+++++++++++/-Tmp-/PKInstallSandbox-tmp/Root";
NSLocalizedDescription = "An error occurred while extracting files from the package "MacOSXUpd10.6.1.pkg".";
NSURL = "#MacOSXUpd10.6.1.pkg — file://localhost/Volumes/Mac%20OS%20X%2010.6.1%20Update/MacOSXUpd10.6.1.pkg";
NSUnderlyingError = "Error Domain=NSCocoaErrorDomain Code=259 \"The file couldn't be opened because it isn't in the correct format.\"";
PKInstallPackageIdentifier = "com.apple.pkg.update.os.10.6.1";
}

Mar 1 19:37:43 Kieran-Seniors-MacBook-Pro Installer[173]: install:didFailWithError:Error Domain=PKInstallErrorDomain Code=110 UserInfo=0x1003f17f0 "An error occurred while extracting files from the package “MacOSXUpd10.6.1.pkg”." Underlying Error=(ErrorDomain=NSCocoaErrorDomain Code=259 "The file couldn’t be opened because it isn’t in the correct format.")

Mar 1 19:37:44 Kieran-Seniors-MacBook-Pro Installer[173]: Install failed: The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance.

Mar 1 19:37:44 Kieran-Seniors-MacBook-Pro Installer[173]: IFDInstallController 14EE80 state = 7

Mar 1 19:37:44 Kieran-Seniors-MacBook-Pro Installer[173]: Displaying 'Install Failed' UI.

Mar 1 19:37:44 Kieran-Seniors-MacBook-Pro Installer[173]: 'Install Failed' UI displayed message:'The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance.'.

Things I've done:

1. Redownloaded any update version and tried to use them. 10.6.6 was downloaded 5 times altogether with no luck. I tried using prior versions such as 10.6.1 and 10.6.2, again with no luck.
2. Redownloaded from a different system. That didn't work either.
3. Reformatted the disk. That didn't do anything. I also zero-filled.
4. Ran the disk repair utility

My system ran updates absolutely fine before this. Anyone got any ideas at all?

Cheers