After installing Vista with XP or Windows 7 with Vista on dual boot, you might be wondering where in the world is the c: drive in which your old OS is residing? But when you boot in your old OS, you can see it also. This is not a big problem actually. The latest Windows OS automatically changes its system drive letter as C, its better to use C. But when you access any other OS all the drive letters will be back in your old fashion. 
So if you are wondering wheres your drive, dont worry, Since the drive letter C is already used by the new OS, your C drive is not having a Drive Letter now. So you just need to give a unused drive letter. For that, open Computer Management from Start -> All Programs -> Administrative Tools -> Computer Management. A window pops up with many things on left bar. You can see Disk Management under Storage. Click that. You can see an interface like a disk partition software. In it, you can see all your present drives with its respective drive letters, but wait, you can also see one drive without any drive letter now thats our dear missing drive
Now right click that and select "Change drive letter and paths..." In that, select ADD and there will be a dropdown box having all the available drive letters. You can choose any drive letter you wish and the missing drive will be having that drive letter.
Dont worry about the working of the old OS, in it that drive will be C itself
So if you are wondering wheres your drive, dont worry, Since the drive letter C is already used by the new OS, your C drive is not having a Drive Letter now. So you just need to give a unused drive letter. For that, open Computer Management from Start -> All Programs -> Administrative Tools -> Computer Management. A window pops up with many things on left bar. You can see Disk Management under Storage. Click that. You can see an interface like a disk partition software. In it, you can see all your present drives with its respective drive letters, but wait, you can also see one drive without any drive letter now thats our dear missing drive 
Now right click that and select "Change drive letter and paths..." In that, select ADD and there will be a dropdown box having all the available drive letters. You can choose any drive letter you wish and the missing drive will be having that drive letter.
Dont worry about the working of the old OS, in it that drive will be C itself
