What you can do is actually mount it under the C drive in a 'linuxy' fashion now by assigning it a 'mount point'. I'm going to assume that you are using Win7 or newer.
step 1: Create a directory where you want to fold the space into. This isn't really optional and is one of the limitations.
step 2: Create the partition on the 80gb drive and format it if you haven't already done so.
step 3: open 'control panel' - 'administrative tools' - 'computer management' - 'storage'.
step 4: on the 80gb drive, right click - select 'assign drive letters and paths'. What you'll do is now instead of assigning it a drive letter, you'll point it to that directory you created instead.
After doing this, instead of having a C: and D: drive (or whatever it was) you'll have all the space folded together into a single drive as presented by the OS in a single C: drive. There is a downside to this, mainly being that unless you actively save files to the directory you created (which by reference will put them on the 80gb drive) it will attempt to save them on the C drive.
This isn't a problem if you want to do something like install games to it which creates a consistent place to put them (ie: create c:\games and mount the 80gb drive to the 'games' directory...), but if you have several programs it does require a bit of focus on where you're putting stuff.