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I remember "standalone" ISTA-Ps were on torrents for some old versions. They are basically copies of folders taken from the installed version. Then you just paste them to your computer at standard locations and use beemerboy88's launcher.

You can still make your own "standalone" ISTA-P by installing it to a computer which has more disk space and then copy to the computer with insufficient disk space. Just be sure to uninstall old version first, else it will be a mess and you will never be able to uninstall them.

You also need to take into account that ISTA-P creates multiple temporary folders/files for each session and they may take some considerable amount of disk space. I would not risk using ISTA-P on a computer which has less than 10-20Gb of space left after pasting such "standalone" version.