I have to say (for a student) you show a marked disinclination toward researching some of these questions for yourself - that is (or should be) the main purpose of having a machine for school in the first place
"It's less so a matter of lack of trust between me and my sister"
It's not so much a question of trust, but of communication - since she appears to be the computer savvy member of the family, one might think that she could readily tell you exactly what was installed and how to use it - - call me crazy, but to me that makes more sense than beseeching strangers for help (not to mention speedier)
While livereg is a part of Norton AV, it's presence alone does not necessarily indicate Norton is still installed on a system - just that it was at one time
There are many Norton products, and many versions. While it does perform as advertised, some find it to be an enormous resource hog - as well, uninstalling the product can prove difficult, and may require the use of a dedicated cleanup tool or uninstaller
So, as you might imagine, I'm not their biggest fan either, however I do have a version installed on an inherited machine, and it can be found under 'All Programs' 'Symantec Client Security' 'Symantec AnitVirus Client' - other versions or products might well be situated elsewhere
Also, an AV should be running in 'real time' in order top provide protection - you might invoke the XP Task Manager and examine all running processes for a hint - msconfig's startups would be another area to inspect
I'm not one of those who think Bill Gates is the devil. I simply suspect that if Microsoft ever met up with the devil, it wouldn't need an interpreter.