Actually, it's not recommended without know how to do it since it should be done by a professional or by an experienced engg/tech. Well, before you decide to format, you'll need a Windows cd, if you wish to install Windows xp, you'll need a Windows XP original CD or if you wish to install Windows Vista, you'll need a Windows Vista DVD, in the same way with Windows 7. Note
: Formatting means wiping out everything from your computer, usually C drive so that you'll be able to install another copy of Windows, which means you'll first format the C drive and then install a new copy of Windows, without Windows being installed, you computer will never run, it's similar to a useless or dead machine. I don't mean only Windows, same conditions apply with Linux or Mac.
If you want to try yourself following the instructions, let me give you few links, but it's not a simple and short process, and I won't take the risk if you fail (this is why I'd mentioned that it's a job to be done by a pro). Here are few links from BlackViper.com regarding Installing Windows. Follow the link/s and follow the instructions, there might be several pages, so make sure you scroll down to the bottom of the page/s and click "Next Page" if you are done with the first/previous page. Before you begin, please read the full tutorial/s so that you will know what exactly you're going to do with your computer. If you can still boot up your computer, make sure all the files are moved from C drive to another partition or I would recommend backing up your computer's files.
1. Windows XP install guide.
2. Windows Vista install guide.
3. Windows 7 install guide.
Let me know if you need further help.
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