I am looking for some kind of external CD writer to use to back up files so

I am looking for some kind of external CD writer to use to back up files so I can get them off my computer.
My goal is to free up space so my computer will run faster. My computer is
System Manufacturer: MEDIONPC
System Model: MS-6743.
My OS is Microsoft Windows XP Home Edition
Version:5.1.2600 Service Pack: 2.0
I am looking on ebay. What do I need to look for? I want to buy a good one, but I don?t have very much money to spend.