They're not "running" at the same time, you can just boot into either at startup. To run both simultaneously would require some sort of emulation like a virtual machine, or WINE in linux can handle many win32 apps. Furthermore, compatibility issues only exist because of stupid proprietary software as they won't tell you how it works even when you're trying to fix it. None the less, ya gotta love how much has been accomplished among the open source community to work with non-free software despite having to kinda guess how it works.
The standard end-user has no need for any certain OS as they just simply use a few apps and the platform which they are on is just a psychological barrier. My mom just uses the internet and office and I started her using LinuxMint 8 x64 a few months ago and haven't had a problem. I even made it a pretty blue for her which windows would not have allowed. She is dual booting in case she needs to do something and doesn't have time to consult me but from what I've been told she hasn't even had to pop back over to windows. While windows was hogging resources, lagging the system and constantly saying that she's "low on memory"... not one hiccup with linux. Plus the added security of root, drastically lower # of possible infectious agents (malware, viruses, etc), and even making it harder for the 14yr old next door to hijack your system and nuke it.
We actually stand around the antivurus displays with the Mac users just waiting for someone to ask
