LTspice XVII runs on 32- or 64-bit editions of Windows 7, 8, or 10. Windows XP is not supported. Windows XP users can run LTspice IV1, which is still be available in observance of Linear Technology Corporation's zero obsolence tradition.
LTspice XVII can be downloaded from http://www.linear.com. A direct link to the distribution file is
http://LTspice.linear.com/software/LTspiceXVII.exe
The file LTspiceXVII.exe is a self-extracting gzipped file that installs LTspice XVII.
LTspice XVII is updated often. After LTspice XVII is initially installed, you can use a built-in update menu command that will bring your installation to the current revision level if you have access to the web. The update process will first download a master index file from Linear's website that has the size and checksum of each file in the distribution. If a file is missing, is of a different size, or differs in checksum, then that file will be updated. Component databases are merged in the update process so if you've added devices to your installation, those additions won't be lost when you run the automatic update utility.
1] LTspice IV is no longer updated.