I am not entirely clear on whether your issue is installing the C++ tools for Windows XP or modifying your project/solution to target that platform. Either way, the following link should help you out.
Here is a summary on how to install the toolset:
- Bring up the Visual Studio installer (Tools -> Get Tools and Features)
- Select Desktop development with C++
- Select Windows XP support for C++ from the Summary section
- Click Modify
