Hi praveen99an and welcome to bytes.com!
To give you exact instructions, we'd need to know which two OSes you're trying to install parallel to each other; however I can give you some general information which might help you:
What you're attempting is often referred to as a
dualboot system, so searching for that term should bring up a lot of information. Generally speaking you'll need (at least) two partitions on your hard drive or two (or more) hard drives. You then install each OS to a separate partition/hard drive. Then make sure you have a boot manager that supports both OSes (usually one of the OSes comes with such a boot manager) and you're good to go.
A few more specific tips:
- If you're installing two versions of Windows, install the older one first as the newer one will come with a boot manager that supports both versions.
- If you're installing Windows and Linux, install Windows first as the boot manager which comes with Linux (usually grub or lilo, though the latter is pretty rare today) supports both Linux and Windows while the one that comes with Windows doesn't.
- If you're installing OS X and something else, check out bootcamp.
As I said before, if you give us more detailed information we can help you with more specific instructions.
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