If a standard reinstall doesn't work, there might be leftover "ghost" files blocking the process. Adobe provides a Creative Cloud Cleaner Tool
Some users may encounter issues with the Amtlib.dll file, which can prevent them from using Adobe Illustrator CS5.1. Here are some common problems:
It is tempting to download a replacement amtlib.dll from "DLL provider" websites. This is highly discouraged. These files are often bundled with malware or are the wrong version for your specific build of CS5.1. The safest method to recover a corrupted library file is to use the original Adobe CS5.1 installation media or the official Adobe Support structural repair tool.