unless you are picky about exactly what components are used in your PC , or you'd like to gain some experience and familiarity with the hardware, it may actually be cheaper (both in time and money) by just going with Dell when they have some good deals with rebates and coupons. Then you'll get a computer with warranty that's tested and confirmed to work together, plus you will get Windows XP as well.

when building your own, you have to order all the separate parts, make sure in advance they'll work together well, and then hope when they all arrive that they are not defective, otherwise you'll end up spending more time shipping things back and forth for replacement and spending time working through compatibility issues.

another thing is that alot of "homemade" PCs are louder systems than those that come from Dell, because they've done the testing and research to know which parts will work well together to produce a quieter system.

that said, building your own can be fun, and lets you build exactly what you want, and you'll learn a little more about how things work and can probably do some light troubleshooting if/when something goes wrong.