Updates not appearing

When I go to the Windows Update site and scan a computer, (this happened
twice today), using XP Pro SP1 it says the only updates available are the
BITS update - SP2 isn't even listed, not to mention any of the recent
critical updates. I don't know how, but on one computer I finally got it to
list all the updates and install them, but on this second computer, that's
all I can get to show, (BITS update), and even that fails to install. I'm
logged on as administrator, and looking at the windows update log didn't
seem to give much information I could digest.

'Any idea why I'm not seeing any updates and why even the BITS update fails?
I can post part of the log file if it will help, or attach it.

Thanks for any info - John