I never turn my WinXP machine off. Instead, I have it set to automatically go to "System Standby" mode after 1 hour of no activity. When I'm done using the machine, I just Log Off and let it go to Standby. When I return (the next day, for example), I just press Enter, wait a few seconds for the machine to "wake up," then Log In.
Recently, I noticed that the machine was becoming very sluggish and unresponsive for a few minutes after I logged in. Using Process Explorer, I saw that a service called "wuauclt.exe" was using 50% of the CPU, which in my Hyperthreaded machine is basically 100% of half the CPU (know what I mean?). The culprit service is part of the automatic Windows Update system. But since I've been using Windows Update for years, why the new slowdown?
After a few searches on the net, I found the probable reason. As it turns out, somehow over the past few weeks (months?) of automatic updates from Windows Update, a new update system called "Microsoft Update" was installed. Microsoft Update not only updates Windows, but also checks on various other Microsoft apps such as MS Office. Apparently, the damned thing does a full system scan for updates every time you log in. Brilliant user experience, no?
Fortunately, it appears to be easy to disable Microsoft Update and return to Windows Update. Just head to http://update.microsoft.com, click on the "Change settings" link, and check the "Disable Microsoft Update" checkbox.
Microsoft seems to acknowledge this problem, and promises a fix in the next service pack: http://support.microsoft.com/kb/916089
