Software Updates "can't be installed" & "failed to mount" messages

Question

In software update; all new software, Quicktime, iTunes, Java for Mac and Security Update; I get a message saying that they can't be installed, followed by :"The digital signature for this package is incorrect. The package may have been tampered with or corrupted since being signed by “Apple”."
I also cannot download software updates (ie: Firefox or adobe flash player). I've already repaired permissions and verified disks. I've also gone to the main websites to download from and when I try to run them, I get "disk images failed to mount.." and the reason being "codec overrun" every time.
Some one please help!

Answer 1

Hi jrsysurfer, and a warm welcome to the forums!
I see you've done some of this already, but since it seems something is corrupted with your OS, am including it all, but I think it may take an A&I.
"Try Disk Utility
1. Insert the Tiger Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Mac OS X volume.
6. Click Repair. Disk Utility checks and repairs the disk."
http://docs.info.apple.com/article.html?artnum=106214
Then Safe Boot , (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, reboot when it finishes.
The usual reason why updates fail or mess things up, or things don't load/run, is if Permissions are not fixed before & after every update, with a reboot... you may get a partial update when the installer finds it doesn't have Permissions to change one obscure little part of the OS, leaving you with a mix of OS versions.
Some people get away without Repairing Permissions for years, some for only days.
If Permissions are wrong before applying an update, you could get mixed OS versions, if Directory is the slightest messed up, who knows!
If many Permission are repaired, or any Directory errors are found, you may need to re-apply some the latest/biggest updates again, or even do an A&I if you have enough free disk space.
The combo update for PowerPC-based Macs...
http://www.apple.com/support/downloads/macosx10411comboupdateppc.html
The combo update for Intel-based Macs...
http://www.apple.com/support/downloads/macosx10411comboupdateintel.html
Repair Permissions before & after re-install, then reboot again each time.
If all the above fails, then it appears to be time for a relatively painless Archive & Install, which gives you a new/old OS, but can preserve all your files, pics, music, settings, etc., as long as you have plenty of free disk space and no Disk corruption, and is relatively quick & painless...
http://docs.info.apple.com/article.html?artnum=107120
Just be sure to select Preserve Users & Settings.
I only use Software Update to see what is needed, then get them for real via...
http://www.apple.com/support/downloads/
That way I can wait a week or so, check the forums for potential problems, and get Permissions & such in order before installing.
If it appears to be time for An Archive & Install, which gives you a new OS, but can preserve all your files, APPs, pics, music, settings, etc., as long as you have plenty of free disk space...
http://docs.info.apple.com/article.html?artnum=107120
Be sure to use Preserve Users & Settings.