Stefan, you've already got a boot disk (likely C: ) and a D: drive. If you direct all of your data saving and downloads to the D: drive, they will not be on your C:. For downloads,create a Download directory on your D: drive then go to your browser settings and point to that folder for the download location.
For all other data, and programs/apps that you install, use the advance install (when available) and select the D: drive as the location for installation. For example, if you go to install a program named whosit, it will likely default to C:\Program Files\Whosit. Change the C to a D and the program will install to D:\Program Files\Whosit, saving you the space on your C: drive. For data, just do a "Save as" and select the D: drive and whatever folder structure you decide on.
For all other data, and programs/apps that you install, use the advance install (when available) and select the D: drive as the location for installation. For example, if you go to install a program named whosit, it will likely default to C:\Program Files\Whosit. Change the C to a D and the program will install to D:\Program Files\Whosit, saving you the space on your C: drive. For data, just do a "Save as" and select the D: drive and whatever folder structure you decide on.