I've seen a lot of documentation out there about the "wuauclt /detectnow" option as a way to force XP's Automatic Updates or Vista's Windows Update to redetect updates.  This is especially useful during a Patch Tuesday cycle, when it's important to make sure that clients are behaving the way you would expect after approving new updates.  However I've had mixed (read: no) luck with "detectnow", so here's how I've forced the detection routine to get started.  The following instructions work on Vista - and should work on XP as well.

• Delete the computer from the WSUS console - it will readd itself during detection
• net stop "Windows Update"
• Delete the contents of the c:\Windows\SoftwareDistribution
• net start "Windows Update"
• From an elevated command prompt, execute "wuauclt /detectnow /resetauthorization"

Note - you may not be prompted immediately to install updates, but at this point when processing is complete (you can watch the updates download and the associated processes working on the new patches from within Task Manager) you should be able to open Windows Update from within Control Panel, and see that you have updates waiting to install.

This will force the Windows Update client to download all updates once again, but it will trigger the installation routine.  Probably not a good idea on production workstations, but this works great in a test environment.