I didn't think the S70 line continued past 2000 (ie: MY98, MY99, MY00 only) - you might want to double check the model year....

If it's a 1998-2000 he Traction control system in that car is called TRACS. There is a TRACS OFF button on the dash near the ignition key area. I don't know how authoritative this is but read it anyway: http://www.autosmanual.com/587/1995-volvo-850-abs-and-traction-control-systems.html

The W button is for winter/wet mode. It just starts the car off in 3rd gear so you get lots of HP+torque going in the engine with low road wheel speeds. I have heard it is hard on the trans to use this mode. It's only appropriate for very low speeds.

If you're interested in a car you should try to find out as much about it as you can ahead of time. For Volvos that you're not familiar with, start with the owner's manuals, which can be found online directly from Volvo for all models:
http://www.volvocars.com/us/tools/OwnersInfo/Pages/VolvoOwners.aspx

FYI I just checked on this site and Volvo does not list an owner's manual for a 2001 S70


*** UPDATE ***
My bad - you said it was a wagon, so you meant 2001 V70. Totally different beast. The owner's info still applies.

There are two traction control system available for that car, and you can have both: DTC, DSTC.

From the user's manual:

4. Stability Traction Control (STC) -option

Press this switch for at least half a second to turn the STC system on or off. An LED in the switch will light up to indicate that the system is on. See page 20 for more information on STC. This system should be switched off if you, for any reason, temporarily have to drive with tires of different dimensions (e.g., spare tire).

NOTE: To help reduce the risk that this system is turned off inadvertently, the switch must be held in for at least half a second to turn STC off. The warning symbol in the instrument panel will light up to indicate that STC is OFF.

5. Dynamic Stability Traction Control (DSTC) - option

This button is used to switch DSTC off. When the LED in the button is ON, this indicates that the system is ON (the light will also come on if a fault has been detected in the system).

NOTE: To avoid inadvertently switching the system off, the button must be pressed for at least half a second before DSTC is deactivated.

The warning symbol will come on to indicate that DSTC has been switched off. The system is automatically switched on when the engine is started. DSTC should be switched off if the steering wheel position or the front wheels are not properly aligned.