There are several ways of doing this. Many of them involve splitting partitions.

I'd advise against this.

If at all possible, when installing Windows, split the disc into several partitions with a view to converting one or more to Linux later, that way you keep your options open.

It's much easier if you split the partitions as early as possible, ideally on a brand new HD.

There is a tiny possibility that you won't be able to boot from past the 1Gb mark (or somewhere) on the disc, but unlikely with modern BIOS/ Linux/ Windows. Many of the FAQs on this subject are out of date in this respect.

Cheers