Test insertion point disappears

I’ve been plagued with this problem since download and install of SP2.

You are typing along and you hear the "boop� error sound and see that the
last characters you typed are not on your screen and you have lost the text
insertion point. You then need to use the mouse to position the cursor back
at the end of the text you were entering and left click before you can resume
typing.

There are five Dell computers, all with XP SP2, in my home. We have all
experienced this situation since installing SP2 a couple of years ago.
Sometimes it happens quite frequently and other times it will not occur for
days at a time. My daughter says she has noticed that downloading and
installing, for e.g. Windows updates, seems to alternately turn the problem
on and off.

It does not seem to be application specific. It happens while using Word,
Quicken, QuickBooks, on web sites, when writing email, actually, it happens
anywhere you may try to enter text.

EarthLink is our ISP and we use Norton AntiVirus. However, I would think
that many people use both of these, and if they had any connection to this,
we would have heard of it by now.

Can anyone can tell me why this is happening and maybe how to fix it.

Thank you