Not to mention, if you build it yourself you have much more choice as to what OS(s) get put on it. That HP, for example, comes with Windows 8, which I wouldn't wish on anyone wanting to do any serious work. Oh, and as far as these OEM computers go, most any brand will give you sub par parts, unless you fork over more money for their server class stuff (which can be quite nice, actually). Overall, if you want to make a cheapo/budget computer, like for ones parents, it is hard to undercut an OEM, but if you want to build a powerful workstation or gaming rig, you can find pretty good savings in building it yourself, and with better parts.
Quick correction to something I said a few posts back, I wrote that stuff about getting a good monitor before your post came up mentioning that you already got an external monitor. If you were in the market for one, then that stuff about looking for accurate color, etc. would have been relevant to the project.