From within XP, you can just create and format a second 10GB partition (run diskmgmt.msc to do this). If you want one 20GB partition, you'll either have to start again (remove existing and create one 2oGB partition with XP installation partitioning tools) or buy something like Partition Magic - which will let you consolidate the unused 10GB into existing partition.
PS. Or you could change the disk from basic to dynamic (also in disk management). Partitions on dynamic disks can be extended (but the basic --> dynamic is one way, you can't convert back).