Big White Rectangle, and sluggish computer

[Tibs]

For about one year (about the time that I up-dated my Norton Security??), I
have been troubled by a "big white rectangle" which, on my screen, measures
34cms wide (full screen width) and 19cms high (approx 2/3rds of screen
height). It appears approx. 36 seconds before Norton shows up, and stays
exactly 5 minutes before disappearing. Until it disappears, it obscures
every programme I open, even tho it's the first thing to appear. I can drag
it down to the bottom of the screen so that it doesn't interfere with my
view. However, its appearance seems to have coincided with a progressive
slow-down of the computer (desk-top) which used to open
tabs/programmes/internet access almost instantaneously. I'm running Windows
XP. Even recently having upgraded from 2mb to 10mb Broadband does not seem
to have improved the speed of internet access, and opening of I/E (Version
7.0.5730.11). I don't know if these problems are related.
Does anyone have any suggestions please?
--
Chris Tibs