Very common problem on your type of vehicle.
Most likely you ABS control unit has failed.
Here's more information on the Volvo ABS issues.
Vitor's web page
Click on the ABS / TRACS Repair link.

ABS control module failure is fairly common. Search on
"abs module", below, to the right. You will find a lot of
posts including options for repair or replacement. If the
module is failing your brakes will continue to work -- you
just might not have ABS.

Ignition switch problems can also cause this. A good general
discussion on other possible causes is at:

http://www.volvospeed.com/Repair/abstrax.htm