Welcome to TechSpot
You need a program like Partition Magic to do that. Fdisk can only create/delete partitions, whereas PM can also merge and resize.
If those 2 partitions are adjacent, then PM can merge them, provided they are the same file-system (e.g. both NTFS).
Otherwise, PM can/will delete the empty partition and resize the exixting XP-partition to include the free space.
However, you would be better off to keep that partition to store your data from other programs. Move folders such as My Documents to that partition. If XP goes belly-up, at least your data is safe.