Opening old excel files

I have a spread sheet application that I have been using for years. I
started with SuperCalc, imported to Quttro Pro and then to Lotus 123 and
later to Excel. I had to make minor changes to the macros and the data
base operations. It has been working great. I sometimes go back and check
previous years. I check 2008 with no problem but on 2007 or previous years
it comes up and says there is a internal file by same name. I type in a
dummy name and it opens but when I save it as the original name then I can
open it after that. I might have gone to Office 2000 in 2007. I am now
using windows 7 and Office 2007. Can any one tell me what is the problem.
charles arnett
cjarnett1 DEL @who.rr.com