No need for a full version, you can use the W7 upgrade, but you need to do a clean install, you can't do an inplace upgrade.
Don't forget to backup.
You use the W7 Easy Transfer Wizard to save your data and settings.
To use the W7 Easy Transfer Wizard:
Browse to the DVD drive on your computer and click migsetup.exe in the Support\Migwiz directory.
Additional XP upgrade help here: http://windows.microsoft.com/en-au/windows7/help/upgrad...