I am running Vista on my computer but somehow ended up with two operation systems when I did a new install. I have a new one on my C drive that I want to KEEP and an old one on my D drive that I want to remove. How do I remove the second operating system? Do you have any DATA or programs on D:? If not, reformat the drive.I have tried to reformat through Acronis and Vista. I keep getting messages that the drive in UNABLE to be formatted. I can't seem to gain access or permissions to reformat the drive. If you have windows disk full install, you can just use it to delete D drive's data.

When I did that a long while back, I had installer MAKE a new partition, which was in other drive.

I don't know / remember what would happen if you simply deleted the unwanted partition and rebooted as is.Thanks for the suggestions

I should make clear that both my operating systems (C, the one I'm using and, D, the one I want to remove) are in separate partitions on the same disk.

I have tried to delete D, but again, I am told I am not allowed to delete system files or am just unable to do so. Also since trying that, I am getting boot errors ("bootmgr is missing"). Consequently I have been having to use the installation disk to repair and boot up. There was no poblem with it until I started trying to remove the unwanted operating system. The system is now acting LIKE it is on its way to crashing. I do have backups fortunately.

Any other ideas would be appreciated.
Nothing difficult is ever easy

Since you have backups, why not re-partition and re-install Windows.

Download the 'tools' from your hard drive manufacturer's website and re-partition and re-format.

Best of luck!Try this partition manager. It worked for me.
http://www.cutepm.com/