You cant feel It
After reinstalling a operating system, it will be more likely that you have lost's the data. However, you could try a recovery data software, this will try and retrieve your data when deleted.
No. You need an operating system to do the installing.
All hard drives must have at least one partition to be usable by an operating system, even if the partition uses the entire drive. If you are installing an operating dissimilar from the one you are using currently (ie. Windows XP and FreeBSD), you should partition the disk from that system's installer, not in Windows.
Service packs generally make the operating system more reliable. Sometimes if you are already having problems caused by corrupt system files or corrupt registry entries, installing (or reinstalling) the latest service pack may fix a lot of that.
Before installing an operating system, you need to consider how big you hard drive is. You also need to consider what types of programs you will be using.
Ubuntu cannot harm your computer's hardware at all. If you dont have much experience with installing operating systems, and try to install ubuntu, there's a possibility that you can overwrite all the data that is currently on the hard drive. However, if you are installing Ubuntu on a blank hard drive, then there is no harm that can be done by doing so. Ubuntu is a very useful linux-based operating system.
yes you do.
Yes, by simply installing a different operating system.
It is not at all important to do a defrag if you just installed an operating system. It is usefull when you have removed software or just before installing new software.
operating system
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No, you do not need to adjust the BIOS when installing an operating system. However, you can set the BIOS to determine the order in which attached devices will boot. That capability allows you an option (one of several options) to install different operating systems on different devices and choose which operating system boots when you start the computer.