IF u seen my thread "new system" then u will see what i'm trying to build.
I just want to know if i can use a hard drive with an operating system already on it or do i have to reinstall it and lose everything?

Most often it won't work, and you will either need to perform a repair or re-install the OS completely. However if the computer you are switching from is relatively similar in terms of components and such, it may work.
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well i'm only switching out the cpu and mobo for now
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The motherboard is usually the determining factor whether or not the current OS will still work. I wouldn't count on it working, and I would back up your data and make sure you have a way to re-install the OS, although it is possible it will still work.
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alright man thanks for the quick help
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