To dual boot click here
No so easy if Vista is on. Also your copy of XP needs to be retail, not one supplied with a previous computer. The licence won't cover installing on your new machine if it is.
But, what programs are you struggling with. I've got some apps that are years old and still work ok with Vista, and one of my Dells is an Inspiron. And I don't understand why you cannot connect through your ISP. This may be just a case of sorting some issues out, rather than taking a drastic step.