|RAM is your computer's memory--the CD-ROM you can simply refer to as a "CD", which means compact disc read-only memory, long-hand, (if I may help with the computer definitions.) :)|
On a more serious note, you had mentioned downloading in your posting, which spawns the question of are you trying to upgrade a computer to Service Pack 2 or 3 from a file downloaded or ordered from Microsoft, or are you doing a fresh install or upgrade of Windows XP?
If you ARE doing a fresh install, and your CD drive "dies" while trying to read the data off the disc in the middle of a read, or the drive has a hard time spinning the disc up (you can hear this audibly on most drives), then the drive is failing, and needs to be replaced. However, on the other hand, if the installation disc is dirty or scratched, then you need to clean it to the best of your ability and reinsert the disc.
If you are upgrading to a newer Service Pack, then you can download it from Microsoft or Microsoft Update and install it through there from your hard disk. You don't need an Service Pack installation CD in this case.
Either way, I hope that I've answered to your question satisfactorily.