motherboard and cpu died

Clarifications

If retail you could conduct an in-place repair. This will preserve your data and programs, but you will need to re-install all the updates.
If OEM and not manufacturer specific you will likely only be able to conduct a fresh install. In that case you could install it into the same partition and recover your data. Blow away the old install and reinstall all your program. Or use the external drive to transfer your data into the fresh OS.
If manufacturer specific, and the motherboard has the same chipset you may be able to convince Microsoft to re-issue a key code when the original fails.

If you are trying to boot into your old operating system using external case via laptop it wont happen, windows will look for same hardware configuration, if it doesn't match the info held by windows you will simply get the blue screen / keeps rebooting.

you can excess the drive as secondary / removable media if you plugin through usb adapter, copy the data.

(I'm assuming it has windows xp) If it is oem (if it came installed on your computer, it probably is), you will probably need an oem cd, if it is retail, you will probably need the retail cd. Boot to the cd, and do a windows repair. (Not repair using the recovery console, but choose to install windows, then choose to repair your existing installation.) Of course, the cd needs to be for the same version of windows--home, pro, or media center. Getting the cd might be the hardest part, because many computer manufacters do not include a normal windows cd, but just their own recovery cd that does not give you the repair option. I think you can still buy the oem cd without a license from places like newegg. Simply use your existing windows product key (usually on a sticker on the side or back of your computer). Doing a repair will fix your driver problems and install the new drivers, or at least the ones necessary to get windows to boot. Windows will need to be re-activated, which may or may not work automatically over the internet. If it doesn't, simply choose to activate it by phone. You may need to answer a few simple questions like "Is this installed on any other computers?" and "Have you made any major hardware repairs to your computer?" (Which is of course the problem in this case.) They will then give you a new activation key. (Replacing your motherboard and repairing windows like this is not illegal or anything. It's just a feature built into windows to help prevent people from installing it on an unlimited number of computers.)