Custom built computers, or computers which have been upgraded/repaired, are all subject to EU laws on CE marking and WEEE regulations. I cannot stress enough how badly this works for the I.T. industry!
If a small computer shop makes a "custom built" computer for a customer, that shop will not have the money, time or willpower to send the computer for a "EMC test" which will cost about £500 – £1000! Considering that the profit made on a PC is around £50 – £100, this is unacceptable. Additionally, even if you build a single unit, you are classed as a "producer" under WEEE regulations and have to subscribe to a "Waste management scheme" – even if you arn't product much, if any, waste!
There should be a minimum limit (probably classed as weight) that you need to be at before WEEE regulations apply to you. Futhermore, custom built or upgraded computers should not be classes as "new products" so an exemption to the law should be made to the IT industry regarding CE marking.
Of course, laws should still be in place that make sure that the actual components used to build the PC are safe (or are themselves CE marked), however these components are made in large quantities by multi-national coroporations.