"They can build me a good spec, complete PC with Windows 7 software (with a few extras thrown in for free) for less than I could buy the equivalent items"
Not surprised
Just remember that for the supplier there is virtually no margin in building a new PC - all the profit is in after sales support / warranty / extras.
Where I used to work (the biggest UK assembly plant) we made more profit out of a ream of paper than we did on a full PC system. Then we went bust..... (with all the money disappearing in Dubai, but thats another story)