I use loads of open source software (Thunderbird, Firefox, OpenOffice). As Wolfeymole says, they have nowt to do with whether or not you are running XP. You don't need to remove anything, so (for this reason at least) you don't need to back up.
On the other hand, if XP was pre-installed, you should at least have a recovery partition. Most (but not all) manufacturers who supply systems without a CD at least supply a utility that allows you to build your own recovery CD from this partition. Check the documentation that came with your PC.