I have an old PowerBook G4 12' with 1,25 GHz and 786 MB RAM, actually MacOS X 10.4.11 is installed. I'd like to give it to my son, but he likes (among others) to use learning games/software, which are made for Windows (.exe).
When I used my PowerBook G4 as my daily machine as a student(2004-2008) it was do-able to run Windows XP within VirtualPC but you needed to tweak the heck out of it. VPC+XP needs 512 MB and up the virtual VRAM to 16MB to improve emulated graphics performance at full screen.
Other than that you can't multi-task with OS X software as performance will take a massive hit. There have been DOSBox PowerPC ports but your mileage/performance varies, you can run Win95/98+Plus Pack and Pinball is playable at a much better framerate vs VPC's uglier overhead with XP.