I have no idea what you did so I don't know why it won't work - but I suspect you didn't follow the correct procedures and ended up overwriting one OS with the other and now have only one installed (or they are both installed but on the same partition and thus can't be separately accessed). There are many other possibilities as well. It's very hard to tell without knowing what happened and how you made the attempt.
Here's how it needs to be done: http://www.vistax64.com/tutorials/224397-dual-boot-installation-windows-vista-xp.html. If you did something different, then you may need to start from scratch to do it properly (depending on what you did). If you can explain what you did (in as much detail as possible and how it differs from the procedure in the above link) and the current situation, it would help a great deal in trying to find the best way to get from where you are to where you want to be.
You should be able to if you have the proper programs installed (like .NET Framework 2) on XP. I suspect EasyBCD isn't working with XP because something isn't there that's needed for it to work. See Method 2 in the above tutorial and it shows you exactly how to do it and even uses EasyBCD to do it and has the instructions drafted so they coincide with using EasyBCD (so I'm sure it works with XP if XP is updated (you may need to install the Service Packs as well) and you've done the necessary preparatory steps).