Partitions at Windows install screen

I currently run XP Home with an NTFS file system. I currently have 5 Gb (which holds system files), 20 Gb and 55 Gb partitions.
When I go to reinstall windows I reach the partition screen. I delete the 5 Gb and 55 Gb partitions without problem, although both are then displayed as 'unpartitioned space' with their corresponding sizes ( 5 and 55 Gb).

From the unpartitioned space I want to create two new partitions, a 10 Gb partition and a 50 Gb partition.

If I select the 55 Gb of 'unpartitioned space' I can create a new 50 Gb partition.
That leaves me with the existing 20 Gb partition (that was never modified), a 50 Gb partition (that was just created), 5 Gb of unpartitioned space and another 5Gb of unpartitioned space.
I cannot howeverr, find a way to combine the two 5 Gb's of unpartitioned space to create a new 10 Gb partition. I can select both of them and create two sepearate 5 Gb partitions, but I cannot find how to create a single 10 Gb partition.

What am I doing wrong? (I hope I wasn't too confusing )

 

those are still listed as logical drives. If they have letters, then they need to be removed. Delete logical drives.
RADIFIED FDISK Guide: Doc's Hard Drive Partitioning Tutorial

 

They're not listed as logical drives. They have no letters and there's no option to delete them etc. They're just listed as 'Unpartitioned Space' with the option to create a new partition from them. They don't seem to be able to be modified in any way, other than to create smaller partitions from the exisitng set of unpartitioned space.