(updated September 25, 2010)
dmg2iso is a WINDOWS utility to convert Macintosh .dmg disk image files to .iso images that can be used in Windows, for example when setting up PearPC.
Note that dmg2iso is actually obsolete. It has been superseded by dmg2img. But I sort of like the old program.
Note also that if all of this is too complicated, you can just use UltraISO to do your conversions. It's commercial software and maybe a little bloated, but the free trial works very well.
An easier way is to download a version of dmg2iso with a frontend. Usage is simple: browse to select an imput file (the .DMG file you want to convert). You will then click on “browse” to specify an output file, but you won't actually browse to anything - just type a file name and click “save.” Finally, hit “convert” to finish the operation. The front end uses an older version of the utility, but for the purposes of Macintosh emulation, it should work just fine.