The two Vista O/S loading on boot on C drive and D drive are not a matter of great concern.
WHEN it would be a matter of concern is that if you had a visitor or family member and THEY were NOT aware, they may then BOOT to the D Drive. and of course download onto a drive that has hardly any space, indeed hardly any capacity to start with..I therefore suggest that although all of this post is for information purposes - YOU do, if there is a risk of that situation, take action on this ONE item., at a convenient time.
You may either and of course I do not know your situation -
1. Always ensure that it is you that boots the computer to C for them or
2. Establish a password for Windows on D Drive with your account ENSURING IT IS an ADMIN account that ONLY you KNOW. and disable the guest account.
AND ANY OTHER ACCOUNT in User Accounts in the Control Panel - ON D.
I could have sent you instructions for removing D from the boot configuration data as it is known on Vista.
This was on XP previously called the Boot.ini.
There are a couple of ways to do that.
1. Is from the cmd prompt but is is NOT easy.http://www.sevenforu...it-how-use.html
2. The other is using a downloaded free program called EasyBCD. It is widely used and well respected.http://neosmart.net/...31611FCB41CE5D9HOWEVER the reason I have NOT gone along that route is that to ensure myself that it is the correct course of action would require
me to be absolutely certain, how this has been setup. It appears to me that the BCD would have to be written to C drive, once the D drive was removed from the boot options.
I consider it better to leave well alone, once we have you booting to C.
Downloads and File saves to C.
Space on D, having removed what you put on it.
WE have cleaned up afterwards