Microsoft has received customer reports of reduced performance while checking for updates on Windows XP systems with restricted hardware configurations (typically less than 1 GB of RAM). The Windows Update team has investigated this issue and determined that this was caused by a small number of updates offered through Microsoft Update that contained detection logic that increased CPU and memory use, resulting in reduced system performance. We have addressed the issue with the detection logic. Even with this resolution, the next immediate scan of a system will still show the performance issue, since the detection logic is stored locally. However, after the next immediate scan the cache will be cleared and system performance will return to normal.
Some customers had observed that by disabling the opt-in to Microsoft Update (MU), performance issues were resolved. Microsoft does not recommend that customers disable MU opt-in, as you are no longer able to receive essential security updates for Microsoft products such as Office, Visual Studio, or Silverlight. We understand some customers were compelled to implement this workaround until a permanent solution was found. You can re-enable Microsoft Update athttp://update.microsoft.com/microsoftupdate.
We sincerely apologize for disruption caused by this issue.
The Windows Update/Microsoft Update Team
http://social.answers.microsoft.com/Forums/en-US/vistawu/thread/d57219af-9893-45c1-8b17-c5cbd472da53