I would sell it with the OS which came preinstalled to preserve the machine's full value, using the Product Key on the COA sticker. Burn the ISO from here with ImgBurn at 4x speed, boot DVD, deleting all partitions during install: Vista ISO download
If the new owner wants Windows 7 have them either do an Upgrade or Clean Install or buy a key to insert now at the link at Control Panel>System - then delete all User folder content and change User Account name in Control Panel.
This way when you start up your desktop it won't reject activation, or if it does you can easily reinsert your key.
The way this transpires is that when you activate with a key that has been used previously you're channeled to a robocall which asks you how many PC's this copy of Windows 7 is installed upon, to which you can only reply 1 to get activated. This gives MS authorization to deactivate it on any other PC which starts up with that key.