Windows Update redirect to KB2497281
If you currently do a cliean install of Windows XP, update to SP3 if necessary, then try to update using the Windows Update website, it will redirect your browser to Microsoft's Knowledgebase article 2497281.
This article is full of solutions for Windows Update problems, but the actual issue in this case doesn't have a solution on this page. There's a link to another Knowledgebase article on this page, which describes the actual problem - that the Windows Update Agent is outdated - but doesn't provide a solution, as it just says there is no standalone WU Agent to download, and it will be updated automatically.
If you don't want to wait, which may or may not work, simply download the standalone Windows Update Agent that Microsoft claims doesn't exist: