Excel 2003 files created in VISTA are corrupted when opened in XP

I have rolled back from Windows Vista to XP because of several compatibility problems with applications I needed and other bugs on Vista. I had Office 2003 but now when I open an excel document created under vista always 2003 does no open correclty under XP. I get bugs all overmy documents and find it hard to work with formulas.
Any suggestion about how to fix this?