I have worked in IT for years. Our users are almost all on Windows 7. It is a solid OS. Haven't supported or used Windows 8, but most people I know think it's fine. What I can say from experience is that almost everyone that I have migrated from XP to 7 really like it and transition easily.
Any reason why it has to be a desktop? What kind of budget is there for the computer? What is the main purpose of it? (work, games, research, streaming por... Media?
Dell is probably ok, but they take much better care of business or large organization users than home. Pretty much anywhere you go, you get what you pay for. I am partial to Asus, but that's just my opinion.
I bought an Asus Nexus 7 tablet last year and almost never use my other PCs/iMac/iPad. It has limitations, but I love it. For a couple hundred bucks, it has more than paid for itself.
Anyway, plenty here know as much or more than I do, but I will be happy to help if I can.