Traditionally adding Windows to an existing Linux box is more difficult than adding Linux to an existing Windows box, because the Windows installation makes an assumption that it is the only game in town and does not look to avoid overwriting the master boot record making it difficult to access your original Linux installation. I have not directly tried this since the heydays of NT 4.0, but I have to tell you it was an ugly experience.
If you do decide to give it a shot I would recommend that you back up anything of importance to you and I would suggest splurging on a commercial utility such as system commander to help try to intercept windows efforts to take over you box during installation. For more information on system commander take a look at:
Another possibility, if you only anticipate using Windows occasionally you may want to consider installing VMWare. While this is also a commercial application, it will allow you to run a Windows instance from your Linux install. Plus if memory servers they allow free trials, so you can try this solution to see if it will meet your needs before you commit to it.
Good luck on your efforts.