Hello NipunK, how are you?
Welcome to the Microsoft Community!
My name is Paloma B., I'm an independent advisor, I'm here to help you in the best possible way.
I understand that you have trouble installing Windows 10 on your computer.
Please check if following the exact procedures of the thread below the problem still persists.
http://answers.microsoft.com/en-us/insider/wiki...
I hope this can help you, just a suggestion, you mentioned your a very old computer that still used Windows Vista, check with the manufacturer's website, if you have drivers for Windows 10, if not, you may have problems with try to use Windows 10.
I await news.
If these procedures helped you in any way, please click on "I solved my problem" and also mark as an answer, so you can help others users.
We will always be available for whatever you need!