Replying to Buying New Computer Help

Okay so my friends family is getting a new computer and i was just wondering nowadays would it be better/cost efficient to build your own, or to buy one already built. The thing is, i can get all of the software and shit for free (XP, Office etc.) I've built a few computers but they were more for gaming, not the family of a budget around 600. So my question is, would it be better to buy parts and build it, or buy a pre-built one.