Ever since Microsoft pushed Version 5 of Windows Update onto my XP SP1 system, I have been unable to access Windows Update - until just now. I found it!! I just accessed Windows Update and successfully downloaded and installed updates to my system.
The particular error message I was getting was 0x80072F78 (appeared when Windows Update tried to find the updates for my system), but I think this solution will fix the problem on systems getting other error messages as well. If your error code starts with "800", I think there's a good chance this solution will help.
Create a file on your desktop called wu5_fix.bat and put the following lines in it. (Leave out the dashes on the top and bottom - I'm just putting them there to separate the batch file contents from the rest of the text.)
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net.exe stop wuauserv
cd /d %windir%
rd /s softwaredistribution
REGSVR32 %windir%\system32\wuapi.dll
REGSVR32 %windir%\system32\wuaueng1.dll
REGSVR32 %windir%\system32\wuaueng.dll
REGSVR32 %windir%\system32\wucltui.dll
REGSVR32 %windir%\system32\wups.dll
net.exe start wuauserv
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[[16-Feb-05: This batch file was edited to include an additional Windows Update DLL (wuaueng1.dll). The problem recurred on my system, and the original batch file I posted here didn't fix it. I looked to see if a DLL was missing and found this one. Adding it solved the problem. I also removed the line registering msxml3.dll since it didn't seem necessary. -sc]]
Now open a DOS box and navigate to your desktop, where the batch file is. (If you don't know where your desktop is in the directory structure, then just put the file somewhere you can find - it doesn't matter where you put it.) To execute the batch file, just type the file name at the command prompt (WU5_FIX). Answer "yes" or "okay" to any prompts. Now try Windows Update again. I'll bet it works. This fixed it for me, and I'd tried everything under the sun before this.
This solution is not in any Microsoft Knowledgebase article I've seen (and I've read over a dozen on the topic), nor did the Microsoft Tech Support person I've been corresponding with suggest it. I found pieces of this solution in different forum posts on the internet, and I'm posting it here because it was such a pain in the neck to find. I hope this information helps someone else with the same problem. There are too many critical vulnerabilities in Windows to not have access to Windows Update!
Please post if this fix works for you - I'm curious.