Can you run OS X on Windows? Yes, apparently you can with PearPC. Is it legal? Well, that is another question on its own.
While it is possible (which is what you asked), there are certain limitations:
While the CPU emulation may be slow (1/500th or 1/15th), the speed of emulated hardware is hardly impacted by the emulation; the emulated hard-drive and CDROM e.g. are very fast, especially with OS that support bus-mastering (Linux, Darwin, Mac OS X do). A lot of unimplementated features are fatal (i.e. will abort PearPC). Timings are very still a little bit inaccurate. Don't rely on benchmarks made in the client. PearPC lacks a save/restore machine-state feature. No LBA48 (but LBA). Currently no support for hard disks greater than 128 GiB. Disks > 4GiB are not tested very well.