Error while trying to install xls tool extension

I can't install xls tool extension. I have a windows xp pc. The error says this:

An error occurred while collecting items to be installed
session context was:(profile=KNIMEProfile, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=).
No repository found containing: osgi.bundle,org.apache.poi,
No repository found containing: osgi.bundle,org.knime.ext.poi,
No repository found containing: org.eclipse.update.feature,org.knime.features.ext.poi,

Is there someone who understand this erro?

Than you.

Hmm, that's unusual.  Could you try again?  It may have been caused by a transient network issue.  

I tried again a few minutes ago.

As you can see from the snapshot, I've enabled all the knime software sites.

Aaron, you were right. I succeded to install somthing yesterday. A few hours ago again was not accessible, it works again.

Thank you for answering my post.