which drive letter are you trying to format?
if you've just added a new drive to your existing system ... and your old drive is still c: (with the new being d, e .... fg....)
windows will tell you you can't format the system drive
(what exactly is the message?)
if this is the case ... easiest is to move your data files (not program files) to the new drive to clear some space
or go the whole hog and shift my documents
http://support.microsoft.com/kb/310147
any new proggys or files ... install or save to the new drive ...
defrag c: ... run disc cleanup and it should be ok
alternatively
to reformat the drive you need to re-install the drives so that the new drive is at the end of the ribbon cable (master - active) and the old is in the middle (slave)
(if you are using sata --- you'll need to config the bios)
then reinstall windows to the new drive .....