Well, the usual suspects would be Best Buy or Staples...or even Wal-Mart.
All pre-built systems are the same quality (junk). But, sometimes junk will do the job.
If you have to buy pre-built, and you aren't a gamer...then you will be buying something with a built-in video chip on the motherboard, which is fine. But as far as "integrated" graphics goes, ATI kicks nvidia's butt. There is much debate about discreet graphics cards, as to which is better...nvidia or ATI. But for integrated chips, ATI has a clear performance lead, it's not even close.
The vast majority of new systems come with WAY more RAM than you will ever use. But you want Windows 7 HOME PREMIUM (note the HOME PREMIUM part, don't get suckered by the Starter /crippled edition). And to run Windows 7 Home Premium, you need exactly 2GB of RAM or more. You shouldn't have to worry, your system will likely come with 4-8GB of RAM, and you will never use more than about 1.3GB*, plus whatever the integrated video chip uses. (maybe another 256MB or more, on average)
For any system with integrated ATI graphics, the processor is going to be an AMD Athlon something or other, which is great. Just make sure that it's dual core (or triple or quad) and at least 2.4GHz. Oh, and do NOT get a Sempron or a Celeron processor (they are both incredibly crippled)
All pre-built systems are the same quality, none will likely last better than any other. But in terms of value, you should look at Gateway, which is made by Acer. Gateway/Acer are not better systems (same quality as all the others), but they are usually better value, in terms of bang/buck.
The following look good:
Oh, I forgot to mention, avoid systems with built-in Intel graphics chips