^ What he said - providing it's a fairly modern PC.
If getting a laptop, then bear in mind that graphical grunt in a laptop costs more than the equivalent in a PC, and that you can't upgrade it in the future. Anyway, the starting place is Zwift's specs - see here:
http://zwift.com/what-is-it/supported-platforms/
I think you could probably get something to meet the min-specs fairly cheaply (eg this Acer for £360 -
linky) - in fact pretty much any new laptop with discrete graphics should meet the min-spec. Ideally though, you're after something that meets or exceeds their recommended specs and decent specced laptops aren't cheap. A quick scout around Dell's site suggests that their cheapest offering that suits is a 15 inch Inspiron for £750 -
linky.
Do look around though - all the major manufacturers will have something in their lineup that suits - just check the specs against Zwift's recommendations. Meeting the processor and disk space requirements is easy, and the memory is easy to check (ie 8GB or more). Comparing the graphics card is the tricky bit - luckily notebookcheck.net has a list of graphics cards in order of prowess -
linky - just make sure the one you're getting is as good as / better than the NVIDIA GeForce GT 650M - eg the Dell above has a GTX 960M which is higher up the list (better).