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I can honestly say that I have never come across this issue, I have 4 machine here all running XP and VS4.5.1 and all are displaying the Vshield icon in the tray.
This happens all the time on the machines at work - and not just on XP.
I beleive its when the download of new sigs/components is blocked (ie firewall at night). It can be temporarily sorted by killing the process (myagttry.exe on VirusScan ASaP which we use) and then re-running it (%windir%\mycio\agent\myagttry.exe on ours).