Change null values raster values to 0 in QGIS?

I need help... I'm having problems with QGIS. I have some rasters that appear to have null values, even though I reclassified them to obtain a Boolean raster :(

How can I change the NULL values to 0?

Answer

beside @R.K. answer, you can use r.null in grass too.

r.null - Manages NULL-values of given raster map.

EXAMPLES

Set specific values of a classified map to NULL:

r.null map=landcover.30m setnull=21,22

Set NULL-values of a map to a specific value:

 r.null map=fields null=99

i hope it helps you...