Why is CPU usage active even when Word is idle

[tatu]

When Word is running, CPU usage never goes under 8%.

1.CPU usage around 30% @ Word activation.
2.CPU usage around 30% @ when a Document with letters is open.
(background spell check?)
3.CPU usage around 10% @ when Word is idle with a new Document with no
letter is open.
4.CPU usage around 10% @ when Word is idle with no Document is open.

I'm curious what Word is doing at 3. and 4..
For Win users, 4.would probably be an open Word window with no Document
is open.

I've never seen such active CPU usage when other word processors are
idle.

(Office 2004 on Mac OS 10.3.9 G3 800M)