As someone before mentioned, Newegg is probably about as good of a source for tools as any.
Buying Windows is one of the biggest issues for most independents that want to build PCs. The problem is that you (we) have to compete with corporations that pay way less than Newegg's OEM cost on a per unit basis. Most of the contracts they have with Microsoft are kept confidential, but I've heard figures as low as $35 for an XP license. At best case scenerio, I end up paying more than double what they do. And in low end systems, Windows will often end up being the most expensive thing you buy for the computer.
Most manufacturers put in cheaper parts than I would ever touch to begin with (and how do you explain the difference between a $15 PSU and a decent one to a customer with no technical knowledge -- and if they had much knowledge they would be building it themselves). As such, I can't say I've really found a way to make a fair profit on building low-end systems with Windows.
I've had some success with building (or refurbishing) some Linux-based systems for really cheap ($200-400). Higher end systems are where you can do best though, because it's a lot easier to add in $100 for Windows and still make some profit. And I'm sure you know that what you or I could build for $1000+ will be massively better than what you will get from Best Buy or Dell for the same price. I recently built someone a gaming system for around $1300 and it included an 8800 GTS. He was looking at systems for the same price elsewhere that would barely have been any better at gaming than the slightly upgraded three year old machine he already had.
Also, you might try to focus on repairing PCs more than building. About 80% of what I do is software and non-integrated hardware repairs on "big name" computers. There is no real big corporate competition there, and considering how unhappy most people I encounter are with places like Geek Squad, I'm not sure there will be anytime soon.
I hope this helps in some way, and best of luck to you. It's hard to compete against corporations that can literally change the landscape of the playing field to suit their needs. But if you find the right niche(s) you can do pretty well.