Not QUITE that easy, but almost. First, look for excel.exe. You should have two. Right-click and hit properties. Find the 2000 version. Note the location.
In Windows Explorer, hold down the shift key and right-click an Excel file. Hit Open With. Hit Browse and go to the 2000 executable (only one version of Excel will appear in the list of programs, and it will be 97, that's why you have to browse to the executable). Choose the 2000 executable. Checkmark the "Always use this program..." box. Hit Ok.