Then if you go with a store bought machine then dont worry about brand but more of the specs.
First off if you have a costco store in your area like I do, they add a 2nd year warranty for free on the computers so I tell my customers who want a laptop and such to get them there just for that reason, otherwise all store machines normally come with only a 1 year.
The system you get you want to make sure is at least a quad (4) core CPU, each core is like a brain for the system, so while 1 program is use one core you have 3 more for other programs allowing you to multitask much better than the old days of a single core cpu.
As for memory and for a normal use machine 4GB or more and you are fine. No need to get a system with 16 GB or anything like that.
And for hard drive, SSD is dang fast, if you can find a system with a SSD drive it is worth the extra cost, we are talking 10x to 20X the HD speed.
As for size of the hard drive, look to see how much you use now, no need to pay for huge drives when you will never need that kind of space.
Other than that keep an eye on specs and try to find a good deal
I didnt mention video as they all have a default and you only need a killer video card if you are a gamer
Shane