Hello and welcome to Tom's Hardware Forums.
XP is a well-developed and, hopefully in your case, up to date software package - Virtual DJ 7.0 is free mixing software. Which do you think could be at fault?
It's always worth getting another download to see if that helps but if it argues with the Kernel or basic heart of XP's programming, it could be because it's been brought up to date to work better with Vista or Windows 7. If that is the case, try using a compatibility mode.