We change our operating system after sometime using one version of windows.During Installation we face many problems about our boot systems .Sometime we are trying to install latest windows like Windows 10 .Windows 10 needs your boot system should be of GPT(GUID Partition Table) for UEFI(Unified Extensible Firmware Interface) and Windows 7 needs MBR(Master Boot Record) system.So we have a solution for you for that problem.We can solve this problem without any OS installed.You just need to download this software called Partition Guru and follow this steps :-
1.Install and run this software
2.Plug in your Pen Drive and open the Partition Guru.
3.Select Pendrive in software and right click and select option Convert to HDD .
4.Accept all the option during making pendrive bootable.
5.Reboot your PC and Boot From Pendrive,
6.Right Click on your Hard drive and click the option convert to GPT or MBR.
7.After All this process then click Save all.
Your Boot System is Changed.