i would like to backup all the files that
we have on the computer. we have many
blank dvds so we will use them for backup.
each blank dvd is 4.7gigs in capacity.

the total files to be backed up is more than
20gigs. i want to preserve the way they are
arranged.

i want to know what software can i use so i just tell it to
backup a certain set of files and i just have
to insert the blank dvd, NEXT dvd, then the next
till the all files are backed up.You need a burner utility that spans discs such as the one suggested at this google hit:
http://www.howtogeek.com/76264/how-to-burn-data-across-multiple-dvd-or-cd-discs/ This INFO is old and may or not still be the same for the software used.
Here is good list:
http://en.wikipedia.org/wiki/List_of_backup_software
Also, here are a few I know about.
 EaseUS Todo Backup Free
 Macrium Reflect Free
 DriveImage XML

I believe all terse have a spanning FEATURE built-in.
You will need to Google to find the right link.
Be Careful! , some software downloads sites have gone postal, they will attack you while offending free software.

IMHO you should only use TOP quality DVDs.

EDIT: You may want to look at what Nero now offers. It is free to try, but after 15 days you have to buy it. Nero has a good reputation.

Nero BackItUp 12 trial
wonderful. thanks given.