"Installation source for MS Office not available"

I have:
XP Pro/SP1
Office 2000 SR-1 Professional


I have got some problems with Excel. When I click an XLS file I get
a message flash up and then vanish which says "Preparing To Install"
and then nothing else happens.

I can launch Excel by running the EXE direct. I still get the
message "Preparing To Install" but then Excel appears.

MS Word is ok.

I decided to do a repair installation. I put my original CD in the
CD drive and clicked Add/Remove Programs MS Office Change. I get
a message saying:

"The installation source for this product is
not available. Verify that the source exists
and that you can access it".

I get the same message if I click "Remove" for MS Office.

And if I go to the CD and list the files and launch the SETUP.EXE
file I also get the same message. (This time the message box is
titled Windows Installer whereas before it was titles Add/Remove
programs.)

How do I get around this to repair my Excel?