I'm not trying to stop you from building your own, but you should probably do a lot of reading and research before you buy anything. You'll want to know what makes up a computer and what each part's job it - trust me it will come in handy when you run into a wall during the assembly of the system. It really isn't THAT hard to build a computer, but then again I've been building them for quite a while so it's easy for me but may be harder for someone else with little experience.
If you do decide to tackle the job of building one, which is highly reccamended, expecially if you have a friend who can help you along the way.
BUT if you decide to buy one, I would advise not to get something from your local chain electronics store (ie: Best Buy, Circuit City)
--- One option is to check out websites that will build a computer from scratch FOR YOU, such as www.ibuypower.com
They build quality systems from the well known parts, just as you would if you were to build your own, bu they build it and configure it for you, and give you just as good of a warranty and support as Dell, Gateway, HP, or the like. This is what my brother did, and his computer has been going for approxamatly a year and a half without a reformat or any harware dying out. They will even do extensive testing (see: Processor burn-in, etc.) on the system before they send it out, just to make sure everything is in working order. They will even organize the cables inside and make your case look reaaaaaly cool!
There are probably other sites, but i have had the best experience with these two. (Note, these sites seem to be pretty much the same company... maybe - they carry approxamatly the same products at the same prices, with similar deals and very similar web site designs.)
EDIT: PS: they have completely customizable systems, so you can get it without a video card or other equipment you already have and want to use and save a bunch of money.