See here:
http://support.microsoft.com/kb/899619/
Also suggestion Pat Walters MSFT:
Please try the following:
Check the My_Computer_Zone has FullTrust permissions enabled:
1. Click Start
2. Click Control Panel
3. Double-click Administrative Tools
4. Click to select the latest version of the Microsoft .NET Framework
Configuration tool.
5. Drill-down to Runtime Security Policy
6. Then Machine
7. Then Code Groups
8. Then Machine
9. Code Groups
10. All_Code
11. Then Right-click on My_Computer_Zone.
12. Click the Permission Set tab
13. Verify and/or change permission set to FullTrust.
14. Reboot the machine
15. Try the installation once more and see if this works for you.
"HiNRG Muzikman" wrote:
> Automatic Update has downloaded but failed to install the new Microsoft .NET
> Framework 1.1 Service Pack 3, 5 times in 7 days. Today, I was working on a
> couple of things during the attempted installation and my computer locked up.
> I had to shut it down. Now, not only is the update not installed, but Media
> Player will not stay open. It says Media Player was shut down incorrectly and
> all "unnecessary plug-ins" have been deleted.
>
> The error message I'm getting for the Service Pack installation reads:
>
> Installation Failure
> Error Code: 0x643
>
> Of course, I can find this error code NO WHERE in Help & Support. Anyone
> have any ideas?