To elaborate for the original poster, there are two obstacles to running Windows .exe files on your Mac. One is that it was compiled for a different operating system. It doesn't just require a different "reader," if you will, like a .doc file, or a .jpg file. Application files are different animals, and in this case they are OS-specific. Second, the much bigger obstacle: they are designed for different processing architectures. Your iBook uses what's called a PowerPC processor. That's what G3, G4, and G5s are. Windows PCs use AMD and Intel processors, which are on the x86 processing architecture. Even if the .exe files were just like any other file, and just needed a "translator," they still would not work on your iBook. It would be something akin to putting Diesel fuel in a car designed to take regular gasoline.
The newest generation of Macs are now using Intel processors, so sometime in the near future, we may be able to run Windows programs on Macs at near-native speeds with either a dual boot scheme (where you can just log out of OS X and log into Windows, and use it like normal), or in a window, like VirtualPC is now. The way VirtualPC currently works, is it emulates an x86 processor; this is why it's slow. In the future, it won't have to do that anymore. Unfortunately, though, the iBooks do not have Intel processors, so that won't be an option for them. I would recommend simply finding a replacement program for whatever Windows apps you wanted to use, or if there simply is not one anywhere, get VirtualPC.