I'm going to paste a portion of a Microsoft support e-mail below. Before I got my two problematic systems to update properly this time around I opened a trouble ticket with MS, and this was a part of their response. These are all maneuvers that I have done before to get recalcitrant systems to update properly. None of my own have ever required this much intervention. However, this time around my two problem boxes had to have their IE caches cleaned and had to be rebooted a couple of times before they would scan the Microsoft update site properly for updates. (BTW, I am using Microsoft Update, not Windows update. I don't know whether or not that makes a difference with respect to the efficacy of cache clearing and rebooting.)
--excerpt from MS support e-mail follows--
Step 1: Verify the local Windows Update services
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You can also first stop the service and then restart them to test the issue.
1. Click Start->Run, type "services.msc" (without quotation marks) in the open box and click OK.
2. Double click the service "Automatic Updates".
3. Click on the Log On tab, please ensure the option "Local System account" is selected and the option "Allow service to interact with desktop" is unchecked.
4. Check if this service has been enabled on the listed Hardware Profile. If not, please click the Enable button to enable it.
5. Click on the tab "General "; make sure the "Startup Type" is "Automatic". Then please click the button "Start" under "Service Status" to start the service.
6. Please repeat the above steps with the other two services:
Background Intelligent
Transfer Service Event log
Note: Event log service is enabled on all of the hardware profiles; this service does not have an option to enable or disable on certain hardware profiles.
Step 2: Check the background programs
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NOTE: If you are running Ad-aware, ZoneAlarm, PC-Cillin, Norton Antivirus, Norton Internet Security, Norton Personal Firewall, Sygate Personal Firewall, Microsoft Personal Firewall, Proxomitron pop-up blocker, Ad Muncher or a web accelerator, e.g. "Slipstream", it is recommended that you to TEMPORARILY disable or shut down any of these before accessing the Windows Update site. Be sure to enable them again when you complete the update process. If you can access the Windows Update site with these applications disabled, then you may need to add the Windows Update site to the applications exception, or the do not block list. Please see the applications documentation on how to do this.
If this does not help, please perform the next step.
Step 3: Clean up Windows Update temporary folder
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One possible cause is that the temporary folder for Windows Update contains corrupted files. Erase all the files there to get the system clean.
1. Click Start, Run, type: cmd and press Enter. Please run the following command in the opened window.
Net stop WuAuServ
2. Click Start, Run, type: %windir% and press Enter.
3. In the opened folder, rename the folder SoftwareDistribution to Sdold.
4. Click Start, Run, type: cmd and press Enter. Please run the following command in the opened window.
Net start WuAuServ
Step 4: Rename CatRoot2 folder
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1. Close all the open windows.
2. Click Start->Run, type: %windir% and press Enter.
3. Locate and open the System32 folder.
4. Find the CatRoot2 folder, right-click on it, and select RENAME. Change the name to CR2OLD. Make sure you press Enter after typing in the new name, so the changes are executed.
5. Close this window.
6. Go back to the Microsoft Windows Update Website and check the result there.
--end excerpt from MS support e-mail--
Of course if this doesn't help you can always open a trouble ticket with MS yourself. There is no charge for it. They provide a link for the contact on the update sites, and they respond within 24 hours. The e-mail I received also contained instructions for sending them the "windowsupdate.log" file and the output from the system info utility so that they could help me diagnose the issue if the above steps didn't fix my update problem.
You could also post more information about your situation that might help people here figure out the problem. For instance, from what you said I'm not really sure whether the browser is not scanning properly for updates or whether it lets you see them and lets you try to install them but the install fails, or just what the problem is with your particular system. If you provide some specific details, including the exact wording and parameters of any error messages you receive, that might give someone enough information to give you a more exact approach to solving your issue.
Good luck!