Un-installing XP is not as simple as just deleting the partition anyway. There is further work after doing this, including reconfiguring Grub so it takes into effect the changes to your OS' or you could end up with a system unable to boot any OS.
You can remove your XP partition from either windows, or from within Linux.
If in Windows 7, hit start, and type disk management and hit enter. The disk management console will then open, and will show every partition on your hard drive(s). You need to exercise extreme care before going any further, but essentially you need to idenitfy your XP partition, and then remove that only. You can then create a new parition once complete to give you the disk space back.
The other method is through Ubuntu. Click System, and then go to administration, and select GpartED. The proceedure is the same as above, except for formatting as NTFS instead of the native Ext4 partition used by Linux.
Remember to then update Grub when your finished with above, but running the following from terminal:
That'll clean it all up for you.
EDIT: The linux method assumes you have GpartED installed, I always have on every install, but I forget its not a default install. To install GpartED fire up terminal and type:
Code:
sudo apt-get install gparted