I'm sure that what you want to do is possible, but I don't know of a site that documents how to do it. The opposite procedure (converting a BIOS-mode Windows to boot in EFI mode) is described here. Perhaps you could work out how to do the reverse based on that procedure -- but you'll need to figure out how to get the BIOS-mode boot loader installed. As a practical matter, if the computer is new, it's likely to be easier to wipe the disk and re-install Windows from scratch.
Be aware that the recovery tools provided by OEMs typically only support one mode of booting (BIOS or EFI), so you'll probably need a generic retail installation disc as a backup for when you need to re-install Windows.
One more point: If you wipe the disk and re-install, be sure to wipe all of the GPT data. Microsoft's partitioning tools erase only the MBR and leave most of the GPT data intact. This confuses libparted, which is what most Linux installers use. The result is that the disk appears to be completely empty (no partitions) to the installer, and if you install that way, you'll destroy your Windows installation. To wipe all the old GPT data, read this question/answer about how to do the job destructively with GParted (you'd do this before installing Windows); or you can use a tool like
gdisk, which can either wipe all the GPT data or convert from GPT to MBR without data loss (which is useful if you install Windows and then discover you've got the problem).