OS Selector and Win7 (not seen)

I tried to follow the instructions to create a dual boot system on a machine that already has Win7 on. I failed and so wanted to revert to my previous single boot system but
1) I cannot deactivate OS Selector (v11.0 build 3,024)
2) OS Selector says "cannot find an operating system" even thoiugh it shows a Win 7 icon that can be selected to boot from.
3) Initially, if I try to reboot, I get a message BootMGR is missing (MBR error 3). Use Crl/Alt/Del
4) If I do this it goes round in a circle unless I choose F10 and then select a different disc to the default and I can then boot back to the OS Selector and into Win7.
I want to get back to my single boot system. How do I do it, simply?

I have another problem.
During my unsuccessful attempt to create a dual boot system on a second disk, that disk lost its letter (D:). If I attempt to give it a letter the options available start with E:. This is also seen in Windows Management. How do I get it back to being called D:
I created a backup before doing this so will a re-installation of the backup onto Disk2 reinstall its letter (D:)?

Could I just uninstall Disk Director to remove OS Selector?
Thanks
HH