As a home user I have bought desktops in the last couple of years from both Arbico and Chilliblast and both have been fine. Not sure I would buy from Mesh again in a hurry although that machine has now been resuscitated and is running Linux quite happily.
Desktops seem to best be found either from smaller mail order builders like thetwo above, or from someone like Dell who are huge and set up to cater to the business market – although that may be what you want reading your post and therefore their outlet site may be worth a look. Tablets and laptops are all the rage so tend to be what you find in retail stores. Depending on where you are in the country good old PC World may be your only option if you want to go hands on before you buy. Having said that they were by far the best deal on spec and price when I wanted a laptop for teenagers school work so may be worth a look.
For office duties and web browsing then onboard graphics and sound and 4MB memory should be fine. The 2 we bought both cost less than 400ukp plus the cost of the monitor ( they were upgrades so we already had monitors to use).
I work for a public service currently having kittens about the potential effect of XP vulnerabilities, I wouldn’t risk XP with client data. If you are not gaming then you don’t need a separate graphics card and as suggested above Libre Office is compatible with Office and the Ninite website will give you access to a whole host of free software that will do most things well.
Win 7 still seems to be readily available from system builders if not on retail store computers. I am no more convinced by Win 8 than Microsoft who are rumoured to be desparately working on Win 9, but if user is used ot it and likes it then fair enough.