And leaves so many bits of itself scattered through your registry that if you want to get rid of it properly you either have to go through for hours manually deleting every entry (dangerous if you don't know what something is) or completely reformat your puter.
I just left mine in place as there was nothing running in the Control-Alt-Delete screen from Nortons (sorry can't remember what that's called in XP), and Avast is just fine - I think my registry entries are just empty leftovers with no actual commands in them and I'm not experiencing any difficulties. If it aint broke don't fix it.
I was using Norton 2003 until an update started crashing my puter about three weeks ago, I had heard from Richard Dean of
http://vu.org how good Avast was and changed at once. A few days later Avast spotted a virus which none of the online scanners I used - Panda, Symantec and Trend - could find, and the programmers here rock the way they helped me sort it out
