Are you running XP without SP1 or SP2 installed? If so, you will be limited to around 128Gb of addressabile HDD space.
If you can install the SP2 update to enable your O/S to handle a larger HDD, then you can use XP's Disk Management to make that 21Gb available.
If you have already got data in E: you can create a second partition with the 21Gb which you could use just for data backups or drive imaging.
If you want to just have a larger E: drive, I don't know of any way you can do that without losing your data on the drive. If you have already or can backup that data, then you don't have any worries about deleting E and recreating it via XP's disk management utility.