Just thought I would plug one of my favorite OS'es, and save ass tools:
KNOPPIX
I call it a spare tire for your computer, and it really is.
It's a complete OS that runs from your CD drive. Don't have to install a thing. Reboot and it's gone. You don't even have to have a hard drive in your computer to run it.
It is a Linux OS, but don't let that scare you - it's got all the pretty buttons you want from Windows. Don't worry about typing any commands.
I have often recommended this to people who have severe computer problems. It's great for those waiting for a new hard drive, or recovery disc set form HP or Gateway to come in. You pop this in your computer and instantly have full internet access.
It's a little lacking on games, but it does have an office suit that is compatible with Micro$oft Office.
I actually once ran a computer on this OS for a whole year, before breaking down and repairing windows (that was only because a Windows game came out that I really wanted).
Oh, and one more thing: It's free!
For more info, check here:
Official Knoppix Site
You can download it here
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