Hello
You may try this steps:
1. The solution, in a nutshell, is to disable your firewall. If you’re using McAfee Personal Firewall Plus, F-Secure Internet Security, Norton Internet Security or another Firewall vendor, see its respective support page for assistance in disabling it (temporarily). If you’re using the built-in Vista Firewall, keep reading.
Open the Control Panel and select Security.
2. Select Turn Windows Firewall on or off.
3. Select Off (not recommended), click Apply and then OK.
4. Go back to the Windows Update screen, click the Try again button
6. The download(s) and installation should start. and this time successfully install. If a restart is required, make sure to re-enable your Firewall once your PC restarts.
7. Though this solution will work for most Vista Update errors see the Microsoft search results for the error 800B0100 if it doesn’t work for you
Link:
http://search.microsoft.com/results.aspx?mkt=en-US&q=800B0100However, if the issue persists, Please perform the steps provided below:
Step 1: Stopping the Background Intelligent Transfer Service and the Automatic Updates service....
Click Start/All Programs/Accessories. Right click 'Command Prompt' and select 'Run As Administrator'. Click 'Continue' on the UAC prompt.
At the command prompt, type the following commands, and then press ENTER after each command:
net stop wuauserv
net stop bits
Step 2: Renaming the Catroot2 folder...
Type the following commands, and then press Enter after each command.
net stop cryptsvc
ren %systemroot%\System32\Catroot2 oldcatroot2
net start cryptsvc
Step 3: Renaming the "SoftwareDistribution" folder...
Type the following command, and then press Enter.
ren %systemroot%\SoftwareDistribution SoftwareDistribution.old
Step 4: Registering the files that are used by Windows Update and Microsoft Update...
At the command prompt, type the following commands, press ENTER after each command, and then click OK every time that you receive a verification message:
regsvr32 wuapi.dll
regsvr32 wuaueng.dll
regsvr32 wucltux.dll
regsvr32 wups2.dll
regsvr32 wups.dll
regsvr32 wuwebv.dll
Step 5: Starting the Background Intelligent Transfer Service, the Automatic Updates service, and the Event Log service....
At the command prompt, type the following commands, and then press ENTER after each command:
net start bits
net start wuauserv
net start Eventlog
exit
Reboot the computer and try Windows Update again.
I hope it may help you.
Regards
Sykes