Sprogget,
Firstly the usual blurb in reminding you that you will have to re-install all applications - even those on your ancillary drives because they will typically have their configuration information in the registry where the hive is in the windows system directory.
If you do still wish to proceed you need to use the windows 7 dvd to boot and go into the partition configuration as part of start up - there can only ever be a single active partition so that will be the one you want to delete - it will ask you to authenticate you want to deactivate and delete the active partition and if you do so you may be required to re-type the label/serial number of the partition as confirmation.
Green_Granite is correct, most systems nowadays will allow you to select a different disk as a default disk, in doing so you could buy a cheap hdd and select that as a default and do a clean fresh install - your other disk will still be seen as a secondary disk and you will be able to assisng drive letters to whichever partitions you want on that disk so the config is familiar to you. I would prefer to do this not because I would consider going back to vista after a decision you have made but because hindsight is a wonderful thing - the moment you delete that partition you may realise there was the odd thing you never backed up or did not work.
Whilst disk space is so cheap I would err on the side of caution, if possible - but its not too hard if you want to be bold...