Hi AerBabe,
I've had a play in Word 2000 (version #9.0.7616 SP-3, to be anally precise), and this seems to work...
The trick seems to be to select the entire column of the table before you start; you can do this by putting the mouse pointer right onto the top border of the relevant column of the table so that it (the mouse pointer, not the table) turns into a solid black arrow, pointing downwards. Left-click, and the whole of the column should become highlighted.
Then go into Format, Tabs, select the "Decimal" alignment radio button and enter something in the "Tab stop position" box (ie "2" for a tab at 2cm from the left of the column). Bob's your uncle.
You can also set the tab, and move it around, by clicking on the ruler at the top of your document, but using the Format dialog works...
Good luck!
Chris.