Q: Can I download an ibook (that I want to purchase) to my PC?

No.   When you buy an iBook, below the purchase button are printed the details of what machine can be used.

You can buy and download it to your PC's iTunes library, but unless it's from one of the publishers that doesn't have digital rights management on their ibooks (e.g. o'Reilly) then you won't be able to read it on your PC, you would need to copy/redownload it on your Mac (or iOS device) to read it

If you want to read an ebook on a PC, it is best to buy it from another store, like Amazon, Nook, Kobo, Google, Sony.