Before you buy in a shop, check their reputation by searching online, and check the price online too. I have never bought a desktop, laptop and very rarely even software in a shop, as I've found better performance and/or price online. You can always phone good online suppliers and get similar advice as face-to-face; sometimes much better advice if you go to the right places.
Suggest you ensure it has the latest Windows 7 operating system, but there are very good, free software packages for internet security and office applications (e.g. OpenOffice)so if you're on a budget go for hardware deals. You'll need a modem/router and perhaps a wireless adapter to get onto the internet, and I would suggest an all-in-one printer/copier/scanner to save space and cost.
If you tell forum members what you want to do with the pc, they will give you suggested machines and point you to deals at trusted suppliers. Cheapest is not always best in the long run - computer suppliers can go out of business before your warranty is even up and after sales support with problems is just as important. Buy with the next 3 to 5 years in mind, because upgrading pc's is relatively expensive compared to buying a new one to get the same performance improvement.