Firstly, try to thing exactly how you will use it and how you might like to use it in the future. I know it's difficult but, for instance, if you were thinking that at some time you might want to do some video editing that would have a major impact on what machine would suit you.
Secondly, try to decide how important support and assistance is to you and also what type. So, do you want an on-site warranty, is telephone support which is available 6 days a week something that you would prefer or would you like to purchase the computer locally?
Thirdly, think about how much space you have available and where might you want to use it. Some people like using the computer to play DVD's or as an alternate TV. Some people will have their computers in their living room and would like them as unobtrusive as possible and of course some people will opt for laptops for portability
Fourthly, ask yourself whether you are likely to want to add things to your system in the future.
Sorry about giving you more 'questions'!
In essence if you believe that all you will do is use the computer for some Word Pro and simple Spreadsheets and you won't really want to expand it much you can get very good value by opting for a machine which has sound and graphics built into the motherboard. On the other hand if you might want to do anything with music, photographic/video editing and/or you feel you might want to expand it in the future then getting a machine which has individual components would probably make more sense.
One answer I will give is about 'makes'. You will get many different opinions from people in this forum and they will be all based on their own experiences and are well woth listening to. I have bought machines from different suppliers and have helped numbers of other people when they have been buying. I usually recommend Evesham computers and have never regretted it, and my recent personal experience of some problems on a new machine, which were sorted out very satisfactorily, reinforced my opinon! Their 3 year warranty is good and the support lines are open on a Saturday!
If though, you have a good local computer shop that can be an excellent option, since they should be able to advise and assist you and if there is a problem they are very handy!