Hi,
Your PC is old are you sure that you have a CD burner in the first place?
If you do the easiest way for you would be to copy and paste the files you want to the CD icon. Windows will tell you that you have files to burn and lead you through the process.

First of all you need to make sure that you have a CD-RW drive. Because your PC is old... it may not have one. You can pick one up at walmart or any other computer store for about 40 bucks or so. Make sure you buy one that is ATA (IDE) and not SATA. Because of the age of your PC... you won't be able to use Serial ATA (SATA). Even today burning Cd's for backup purposes is becoming outdated. A CD only holds 700MB while a DVD hold 4.3 GB! A dvd burner won't be much more expensive. And a DVD burner can burn to regular CD's as well.

Another option you have is to buy an External harddrive. While they are more expensive, they provide a Quick way to back up your files, keep the portable, and when you finally get a new computer you can copy your old files easily and MUCH faster than burning to CD's.

If you still want to stick to burning cd's and you have a cd-rw drive... just open nero and select to burn a Data Disc. Add the files which you want to burn and click burn on nero. You can also use the built in windows tool which is just as simple. Simply drag and drop the files that you want to burn to the CD-RW icon in My computer and windows will do the rest.

Hope this helps...

-Firekid