If you did a reinstall with discs instead of the Dell PC Restore, that partition can no longer be used.  I would keep the Dell Hardware Diagnostics partition.  However, if you are so low on hard drive space that you need the small amount of disc space that you will get from the recovery partition you need to replace the drive with a larger capacity drive. 
I have had to do a clean reinstall of Windows Vista Home recently on my Dell Inspiron 1545. This was due to me getting an error message when trying to return the system to its factory state. (I think the PCRestore application no longer works and is corrupted.) Is it okay for me to format the Recovery (D:) Drive on my laptop and remove the partition in order to gain more disk space (I don't anticipate ever using the PCRestore function in the future)? Thanks
Can I format my Recovery Drive?