Windows 98.

Seriously. That thing was an unstable bloated piece of crap that'd crash for no apparent reason in the middle of typing an essay, usually 0.5 to 1 hours after last saving the document.

I was ready to try anything else and found something much better which enabled me to do crazy stuff like burning a CD and playing mp3 files at the same time as typing an essay. Windows 98 would have a hernia if I tried to do that.
Slackware. I've tried others, but Slackware is the one I'm most familiar with. I can't be bothered learning new ways to do the same thing for the same result.

Dear friends,
Remember 2 things:
1) GNU/Linux is not "the other system but free", it's something different, and compatibility issues are not Linux fault per se;
2) Why are we still slaves of the other office suite? ALL the compatibility problems are their fault, since they work FOR complicating things, including lines and lines of code that got messed um not only in Libreoffice and OpenOffice.org. but also in their own (payed) version for the MacOS!!!I advocate to stop using PPT formats, as well as any ".---x" format (docx, etc.) and moving yesterday to ODF!

Cheers!