Especially if "where" is some fly-by-night internet outfit who is selling stuff that "fell off the truck". Sure, it'll pass WGA and Activation -- it's not a pirate -- but it might be a little bit warm to the touch...
OTOH, all my copies of Windows are strictly legal, no hassles, no worries.
> You should be paying retail.
For most people, yes. In my case, I run so many copies of Windows, between the "on-the-metal" native installs, the three Macs that are BootCamped, and a pocketful of VMs, it turned out cheapest to get an MSDN subscription at the "Operating Systems" level. Pretty much have access to everything, all the latest releases. And I have clean, up-to-date installation ISOs of everything back to Win 3.11, should my little heart desire it.
Obviously, I'm not the average case... :)
Which brings me up to the next thing: Somebody follows the advice and does this and starts to build their own system and gets ready to install their copy of Windows and "Oh, sorry, you must have Vista or XP installed to run this update." This means the poor guy who followed the advice given earlier in this thread either has to track down copies of Vista or XP to install BEFORE installing his Win7, or he can buy Windows 7 Home ($179 or Pro for $265 on Amazon.) Undoubtedly, someone will be along to explain how to fool the installation into thinking you've already got XP installed.
The Samsung monitor one of them was talking about happens to be the one I use for a second monitor with my iMac setup. It's fine for holding the palettes and Photoshop and Aperture control boxes and playing a book on iTunes while I work on photo editing, but I would NEVER use it for the serious photo editing necessary to make sales. The image being edited is ALWAYS on the iMac screen.
So much of the advice given here on building your own box is just garbage.