Using Gimp to batch convert images to another format in Windows

Better than Gimp or Irfanview is ImageMagick.

For instance, try:

mogrify -format jpg *.png

There is also a wizard based batch processor available:

DBP (David's Batch Processor) for GIMP

Is using Irfanview not an option? I keep both GIMP and Irfanview installed for precisely this reason...GIMP for image editing and Irfanview for batch conversion.