|You original ASR backup will enable you to restore any computer from it as long as there's enough disk space.|
If you do another one, it doesn't mean it will remove the original - you get to specify where to save the backup to - so just pick another location/file name. Many operations will ask you to redo stuff after a major system change.
btw - it sounds like you're storing your backup on a different partition of the SAME hard drive your system is on. If this is so, its a bad idea, as a disk failure would take both system and backup. Ideally should be on CD/DVD or external hard drive, but at least on a separate hard drive. Also, have you considered using software like Acronis True Image. Not free, but gives you much more flexibility than XP's built in options.