A contrarian view for your consideration.
Building a PC is simple, if you know how. I went that way once and after spending hours, hours and hours surfing various forums for the 'cutting edge' components and best build ... ended up with a wierd glitch where the 'north bridge' on the mother board black screened my pc when I installed my audio interface. It was a good interface,
Echo Audio Layla 24, so I contacted the company. After 3 months they said they were sorry, they couldn't write a driver to make my setup work. Moral of the story ... all the 'best' parts don't always assemble into the best thing.
My advice is go one of two ways.
1) Google something like 'PC DAW,' or go to
http://www.pcaudiolabs.com/,
http://www.sweetwater.com or any other place you like and look at there completed packages. In this case you'll know everything is setup properly to work together optimally and under warranty. You get the DAW and interface all set up and working together. You can even buy with your choice of software installed. Also, because of economies of scale, i.e., these builders can get parts cheaper than you can, you might be surprised that they aren't much more expensive than building one yourself of the same quality parts. (see below the info on audio interfaces).
2) Go to the online Dell Outlet store and buy a 'refurbished, demo or returned' computer. You will save about 30% off what it would otherwise cost and it has an identical warranty as their new computers ... except the unconditional return policy is only 15 days instead of 30 days. I've bought 8 over the years, once had a problem, and it was immediately resolved. Then get some information about audio interfaces ... which you should do anyway. Check out
http://www.tweakheadz.com for a good general guide to what's available and the relative advantages/ disadvantages ... and to cut to the chase ... PCI/ PCIe interfaces will always be the most reliable.
Note, if you build it yourself and something goes wrong ... you have to troubleshoot which part. There is no online 'technical support,' because you have 10 or 12 different warranties on the different parts and they'll all want to blame someone else's components.
Lastly, make sure you understand 32 vs 64 bit. With Win7 you have a lot of good choices. You can run 32 bit programs ... as most DAWs, under a 64 bit O/S. The big advantage is that you can have up to 4GB or RAM available to each program ... provided you have enough RAM. Shoot for 8GB if you go that route so you can have that much for a program and plenty left over for Windows. If you go 32 bit, you have max RAM of 4GB with about 3.7 available and nearly 2 of that hoarded by Windows. In my mind, there is no doubt someone should try to go 64 bit with 8 GB minimum on Win 7.
Good luck.
Prado