|"Is it on the same partition as XP?"|
You never install two operating systems on the same partition.
If you did that, you're probably going to need to install XP from scratch. Installing two operating systems on the same partition usually messes up both operating system installtions.
If you need to do that, then if there's data on the drive you don't want to lose, you will need to boot the computer with something such as a bootable Ubuntu or Linux CD and copy the data you don't want to lose on XP's Windows partition - the partition XP's Windows folder was installed on - to elsewhere BEFORE you install XP from scratch (you don't need to copy the Windows files or anything that you can easily install again).
If Ubuntu and XP are on different partitions, you can install and use a third party "partition manipulation" program in XP to add the unallocated space made available by deleting the partition Ubuntu was on to the Windows partition, and re-sizing the XP partition to use the unallocated space, if the unallocated drive space is adjacent to XP's partition.
E.g. the freeware Easeus Partition Master Home Edition.
It's recommended you copy at least the data you don't want to lose to elsewhere BEFORE you re-size the partition, but usually you won't have any problem.