custom built computers
There are several 'regulations'.

The power supply bears the brunt of complying with many. Mains safety, mains-borne EMI, low power when in standby, power factor. The rest of the computer is at low voltage DC, so doesn't influence these.

Fire risk should be handled on a part by part basis, with them being fused internally or otherwise protected from drawing too much current, or being metal cased so that a burning component in a part doesn't spread to others. 

Transmitted EMI should have also been pre tested part by part, but the connected system obviously cannot be tested for that until it's assembled. Here, the case, with its conductive flanges can improve the system EMI. I'll wager that custom-built computer suppliers don't test each machine they build for radiated EMI anyway, as it's a difficult and expensive test to do properly, but rely on the parts and case.