When you start the backup, where does it say it is putting your DVR files? By default it puts them on your C drive, but this isn't so good because the very drive you need working well for Windows, can get full very quickly.
If you have another hard drive in your computer with lots of space (e.g D) it's best to change the location. Open up their web server, top right icon 'System Setting' then go to General/File Paths and see where the files go or change it to another hard drive.
Because the backup files can be very large, It's a good idea to occasionally run Windows 'check disc' to see if the PC disc is OK before trying to backup files from the DVR.