I would suggest that, while possible, installing Windows 7 to a drive other than C: as the SOLE OS would not be a good idea.
Basically, Win 7 installs the SRP, which then labels the OS partition that is created the C: drive by default. You will undoubtedly have significant issues arise, either now or in the future that will make you wish you had not made this choice.
Just use mental discipline when you install programs and direct them to your HDD, D: E: or whatever, it's really not that difficult compared to what you may face later.