You might get away if you uninstall all of the installed drivers first before moving the HDD over. You should also uninstall and disconnect any non-essential components (WiFi cards, etc) from both machines. Activation will certainly throw a fit anyway, though. You may also need to change the SATA mode in the Optiplex's BIOS to match that of the Vostro in order to dodge an insta-BSOD on the first bootup.

you'll have to edit the registry and change the current control set/services/msachi and all other controlsets/services/msahci... change Start from 3 to 0 IIRC.

or you can simply use Sysprep and let windows install the hardware the right way

Thanks guys, this sounds like the same kind of pain in the butt XP was... Was hoping they'd have made it easier. FWIW both machines have a valid 7 Pro OEM COA so at least reactivating it shouldn't be an issue.

weldum wrote:

or you can simply use Sysprep and let windows install the hardware the right way

I had a look into this, it seems like it'd be helpful - does it disable all the drivers for you too, or would I still have to uninstall them all manually?

Correctly configured, it will delete all the installed hardware, and when you put the hdd on the new machine it will detect and try to install everything for you.
However SysPrep is an utility for creating windows os images for deploying on multiple computers. It is recommended to use it with a clean os. In cases like yours it may or may not work.
Worst case scenario would be that simply running the software would end in an error window.
There are other alternatives, but they are mostly based on modifying the registry, deleting files and changing bios settings.