If you want a PC rather than a laptop then it is very difficult to get up to date accurate reviews.
Technology inside PCs moves so quickly that by the time the product has been reviewed and appeared in the press the components inside have probably changed (different CPU, diffferent motherboard etc).
There are loads of PC magazines that review PCs (pop in to W H Smiths and scan the magazines there).
PC Pro magazine do a lot of reviews, and they do say to companies the PC must be able for a certain length of time AFTER the review to ensure people can still buy it.
They do have some of these reviews on their web site. Here are the labs, but also see reviews of individual PCs on their web site as well
http://www.pcpro.co.uk/labs/