attachments broken (mail), updater brok


I have a MacBook with OS X 10.5.8.
I have two problems.
My mail program no longer accepts attachments. Any attachment I add or that is sent to me is replaced with the phrase "missing plug-in".
The second problem is with my software updater. When I try to update it tells me I have a network problem. I am getting email and browsing fine. When I go to Network Diagnostics it tells me my connection is fine.
I tried re-installing Mail from my OS X cd that came with the computer but now mail tells me the version is not compatible with my operating system. I can't get the updates (and I don't know if that would help anyway).