With Windows XP you have to use a third party program to do it... I use Minitool Partition Wizard Home Edition. It's perfectly safe as long as you don't program a number or operations to be done at the same time particularly when it involves the System partition. I abused the number once (5 or 6 operations) and I had a minor problem so when you're doing a modification to the System partition that requires a restart, just program one or two operations at a time and it's perfectly safe. Modifying non system partitions with this program doesn't necessarily require a restart and it's less likely to cause problems.
http://www.partitionwizard.com/free-partition-manager.html
Install and open the program, it should show the graphical view of the partitions in your hard drive... with a right click, select the partition above the unallocated disk space of 19.6GB, and click on "Extend"... you should see a line that reads "Take free space from" and below it a line that reads "How much free space do you want to take" (from the 19.6GB), so move the slider all the way to the right to select the full 19.6GBs and next press OK.. below on the left pane you should read "1. Operation Pending" with the description of what is about to be done, next press the Apply icon on the toolbar and wait for it to finish... If the partition you're extending is the system partition, it will probably prompt for a restart to complete the opeartion during the reboot... Accept and restart the computer, and check with Minitool or the Windows Disk Manager that the unallocated disk space is gone.