If I install Ubuntu Linux on a computer with an AMD processor and 256 MBs or Ram can I uninstall it?

I was thinking that I would put Linux on a computer that was build by hand a while ago. With Ubuntu Linux I will be able to install Flash Player, right? Also, the computer has 7 Gigabytes of hard drive space, It's not a lot but that's all there was laying around. If I install Ubuntu linux, will I be able to install flash player for youtube? Also if I'm finished with the Linux, how do I uninstall it and go back to Just Windows XP? With the Program Wine would you be able to run stuff that requires XP Service Packs? How much disk space does Linux take up? I'm going from ubuntu.com if I download but click Run instead of Save file, does that run the installation or the operating system?

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