Sorry that your upgrade experience was not a good one but from reading your post it does seem that you went to PC World somewhat under prepared. If you had looked at the components you were intending to keep before you went out and noted which ones were IDE, which ones were SATA, what your graphics card type was, what style of memory you had and your power supply connections then you good have saved yourself some grief.
Building your own PC and even upgrading an existing one is not necessarily the cheapest option as you are buying individual components while the "box shifters" buy in hundreds or thousands. However by doing it yourself you do have the satisgaction of specifying and knowing the quality of the components that you are getting and the experience of fixing it if it ever goes wrong.
Your upgrade has added greatly to your PC knowledge, you will be better prepared for the next time that you upgrade and when you read posts from forum members with upgrade problems you will be in a position to answer them with the hindsight of your own experience.
Thank you for contributing.