If you really have low memory, you could completely clean out your HD and never give you more memory. The reason is that Hard Drives are storage, they are never ever memory.
Your Ram is your memory.
Think of storage as a book, you can set it up on a shelf, ignore it and the information stored on the pages is there when you open it up. Hard Drives are very much like this, but more Fragile. Once the information is there, unless you remove it somehow, it remains.
Memory is like your brain, stuff stays there until the electrical power is removed, then all info is gone.
So, given that you are either talking storage on your hard drives, or you are talking about not having memory the help would be different.
Second Question
Now if it was me, I would first get an accounting of what files were on the second drive. Take a screen shot of the listing, print it out and check off each folder you can get into. I would see if there were any filled with stuff I needed, I would then copy that stuff onto a dvd. If I could not copy my data out of a folder, consider it lost and go on to the next. Once I went through each folder, I would reformat the thing a couple of times. That will deal with anything strange on the HD and prepare it for my stuff.
Then I would deal with the extra stuff on the C drive. Depending on how large your C is, I would begin putting my data files on to the other hard drive. Sometimes the easiest way is to bring up the stuff by using the program that created it, then use the Save As command and choose the new HD as the location. I would also begin to archive the files I have that the various computer programs have either an auto or manual archival command. Like a photo program, or iTunes or other music program anything else I was interested in.
The reason I do this is that if a computer can complicate your life it will, and right now with all the people out there helping other people's computers do strange and wondrous things to their owners and their info, well you need to archive at least quarterly, or monthly. I like dvds since they are so cheap now and take up very little room.
Then you make sure you have a plan on what goes on which device. Keep a neat computer and let the house take care of itself.
Of course this is only my opinion, and I have only been doing this since 84, and before that I was getting my degree in computer science, but that was back when we still used cards and Fortran was the cats meow. Now I am just old and opinionated.