"you chose to skip all updates" error

Hi,
I'm having the following problem on Windows XP workstations in our network.
Windows Update will work normally, apart from one update. When trying to
download that update, the installer screen will say "you chose to skip all
updates". This error pops up immediately. The strange thing is that I can
download and install 30 updates but there will be one that won't download.
Strange is also that the update that won't install is different for different
workstations. Some workstations will not download a very small update, but we
also had one workstation that would not download the whole Service Pack 2 for
XP.

Can anyone help?
We have a Novell network with Dell PCs and Laptops.

Thanks in advance,
Jos van Iwaarden
Holland