To be pedantic, your laptop hasn't got two hard drives. It's got one hard drive which appears to be split into two partitions (or one partition with a virtual drive on it).
Usually you can treat the two 'drives' as if they're just very big folders (each of which is, of course, full of sub-folders). All you need to do is set the relevant programs to use your required default settings. For example, if your browser is located on 'Drive C' it will save all images, videos, programs and other downloaded files to a default folder on 'Drive C' unless you tell it otherwise by going into 'Options' (in Internet Explorer or Firefox) or 'Settings' (in Chrome). You can either specify a default folder on 'Drive D' or, as I would do, select the 'ask me where to store files every time' option.
Similarly your word processing program will probably have an option allowing you to specify the default location for saving files (or it will simply offer the last-used location as the 'first choice' when saving files).
It might be a bit fiddly at first but, once you've set up the defaults for each program (or they've memorised the last-used locations as being on Drive D) you'll have no further problems.
My own choice would be to keep data which is regularly called upon by programs (such as fonts) on the OS drive (which I've referred to as 'C', above) and to keep files created just for use (such as documents, images, videos, etc) on the other drive ('D' in my example).
Chris