This isn't what I would exactly call a mashup, since its just two tracks played over one another ...... BUT that said, it takes a good ear to be a good mashup maker, and this does work. Just one thing bugged me, and if you don't mind, I would like to offer you some advice on how to fix it.
There is a 'wobble' in your Britney track, you can really hear it in the vocals. This comes from stretching or warping the track to fit the BPM / tempo of Planet Rock. Most production software (Sont Acid, Ableton etc) have whats different settings for this (they are called algorithms). If you leave it in the default mode, it will always provide a wobble on things when you've over stretched them. If you let me know what software you're using I can tell you what setting to use to solve any future issues.
If, however, you're using DJ software (Traktor, VDJ etc) - these don't have algorithms (because they are not meant for production).