1. Partition your drive, shrink the xp partition to leave some free space. Then when installing ubuntu use the "use largest continuous free space".
2. Use 2 HDD's, pretty much the same thing as partitioning but the xp drive manager can't shrink its own partition, unlike vista, so this saves having to use a 3rd-party drive manager since when you install a HDD it comes as a seperate partition unless you unite both into a spanned partition. Wipe the other hdd and then install ubuntu to it.
3. Use Wubi to install ubuntu into xp just like a application, also very easy to uninstall but that's where the pro's end. Vista's UAC and other advanced secrity settings can cause a lot of errors and ubuntu is slower at least that's the case in 8.04 (I don't know if wubi improved in 8.10). Also you can't hiberate.
4. Use a networked drive. ONLY FOR VERY ADV USERS AND COOPERATE MANAGERS
Use a networked drive and a boot-loader that supports network booting. In laymen's terms you boot off of a HDD connected by ethernet. Boot from a HDD with ubuntu or install ubuntu to a networked HDD. Of course this rarely is used by home owners.
That's all the dual-boot methods I know.