For users who genuinely need a portable spreadsheet application, legitimate alternatives exist. (via PortableApps.com) offers a full-featured spreadsheet program that reads and writes Excel .xlsx files, supports most formulas and charts, and requires no installation. SoftMaker FreeOffice Portable provides a more Excel-like interface with better compatibility for complex formatting. Even Microsoft itself has moved toward cloud-based portability: Excel for the web runs in any browser without installation, saving files to OneDrive. While these lack some advanced Excel 2013 features (Power Pivot, certain macros), they satisfy 95% of on-the-go spreadsheet needs without legal or security pitfalls.
The Ultimate Guide to Excel 2013 Portable: Risks, Features, and Modern Alternatives excel+2013+portable
Use Microsoft Excel Online (free, cloud-based) For users who genuinely need a portable spreadsheet