The fact that you used this DVD to record some video means that you did burn video files to this DVD.
Now coming to the erase part. If as you say, this is a DVD RW disk, you should be able to erase and re-use this disk several times.
I use "ULead DVD Movie Factory" which I got free (bundled with Hauppauge HVR-1950) to erase my DVD disks. So far, it has worked well (for over 8 months). You may be able to download this program at no cost. If not, there are other such free programs that work well.