Hi Lewis, What operating system are you using? XP, Windows Vista? It's likely that System Restore is taking up quite a lot of spare space on your main system drive (C:). Try selecting the C: drive in My Computer, and follow the following instructions:
1. Right Click
2. Select "Properties"
3. Select "Disk Cleanup"
4. There should also be an option for System Restore, select to delete all but the last Restore Point. This may free up some space as well.
Additionally, unless the files you have stored on the C: drive are for installed programs (i.e. are part of the program itself, rather than just a word document or something) then there's no problem storing them on the other drive. Just select the files you want, right click, select "cut" open up the D: drive, right click, and select "paste". It will then move the files from your C: drive to your D: drive, freeing up space.
You'll also notice that the more space you have the better your operating system behaves. However, NEVER EVER move program files etc. Only files you have created or downloaded are safe to move about.