how to remove the second operating system?

If it is a Linux and a Windows, you can just delete the partition that the system you want to get rid of is. If is the Linux you just will have to delete the Swap and the Ext partitions and let the Windows' NTFS. Or if is the Windows the system you want to get rid of is just do the same process but invert it. Delete the NTFS partition of the Hard Drive and let the Swap and Ext.

Installed 2 OS in same drives ?...Oh ! we should not install like that...There are lot of risks in that...I don't think that you can remove one OS alone since you have installed 2 OS in same drives...so just format your system completely..take necessary backups..and then install new OS..Installing 2 OS in same drives is very bad..and it can make your system work very slowly...

I think as you format your drive normally so format the drive manually or you may try to insert system disk... and try to format the drive .. on which you have installed the second operating system..

sir, the only way is reformat your computer, cause it seems that you installed new OS without reformatting your HD. done this before.. hope it help..:D but remember to back-up important files first by burning to DVD or another HD..:D