2003 will read 2000, so why go backwards? And 2003 will save in a format that 2000 will read. Yes, installing 2000 can mess up 2003. Though it may not let you do it, even if you uninstall, since the program resides on the disk even after erasing, 2000 would just install as 2003 anyway. (I don't know why the disk you have won't read, all office products are backwards comaptible)

When you attempt to install the older version you will get an error message that states "Newer version already installed"

The new Office will read any documents from the older versions. Office documents format has not changed since Office 97. Excel hasnt changed formats since Office 2000. Access has changed formats in 2003 so will continually prompt you to update the database (which then means older versions can no longer access the data).

As Lee stated, there is no reason the discs that came with your Office 2000 books wont work in 2003. So the question stands, are you sure the discs are still good? Are we talking CD's or Floppy disks... Do they work in other computers still? Can you see the files when you browse to them on the CD or Disk? Maybe a read error and not an application error.

I've decided to just study from a 2003 tutorial and live without my favorite computer books. Thanks for the help anyway, guys.