Probably not, you can check your BIOS, but A has been reserved for floppies as far back as I can remember. You can also check My Computer>Manage>Storage>Disk Management and try to change a drive letter to A. You will likely find A is not a choice.
I have 64-bit Windows 7. I just tried change the drive letter of my SD card reader to "A:" and it was successful. "B:" worked too. So it looks to me like this is possible.