Hello granma6, welcome to the forums.
Generally, your C drive will contain your operating system, so you don't want to do a clean sweep of the drive. To make more room, you'll probably need to look in the folders where the videos are stored on the hard drive, select the ones you no longer want, then right click on the file or folder you wish to get rid of. This will bring up a pop-up box of options, one of which is Delete.
Once you delete the file, it will go to the Recycle Bin that shows up on your desktop. To permanently remove it from the drive, right click on the Recycle Bin and empty it.
If you want to save all the videos, you should burn them to a disk or usb flash drive before you delete them off the hard drive. You could also buy an external hard drive and simply move them there.
If you have a desktop computer (not a laptop) another option would be to buy another internal hard drive for more storage space. You could make the new drive the target for you Dazzle program, so that it automatically puts any new videos on this new, empty drive.