I must install Ubuntu over Windows XP. Is it necessary to overwrite all data on the computer to do so?

It depends on whether the Ubuntu's installation program automatically formats the drive you're installing it to as part of the installation process. If so, then unless you put a new drive into the computer and install Ubuntu onto that, or partition your existing drive using an on-the-fly drive partitioning program (such as PartitionMagic), then there's no way to save the existing data. The reason for that is because Ubuntu uses the ext3 or ext4 file system while Windows uses either FAT32 or NTFS--none of which are directly compatible with each other. Ubuntu can read FAT32 and NTFS file systems, and I think you could install it onto a FAT32 or NTFS file system, but it's generally considered a bad idea (especially for FAT32 because of the limitations of the file system--NTFS isn't as bad, but it's not as good as ext3 or ext4). Short answer: it may be possible, but it's probably somewhat difficult to do and definitely not recommended.