Hello and welcome to MobileRead!
The last version of Calibre which is guaranteed to work on Windows XP is 1.48. You can download it from here:
https://download.calibre-ebook.com/1.48.0/
Be aware that this version is pretty ancient and so may not have all the features the newer versions have.
The last version for Windows 7 is 3.48:
https://download.calibre-ebook.com/3.48.0/
If you use Linux, you can download and install a newer Calibre version:
https://calibre-ebook.com/download_linux