Should I use a voltage regulator on a desktop computer?
Switching power supplies act on input voltage variations faster than simple relay based stabilizers (by orders of magnitude). Such equipment should only be required if voltage varies beyond your power source input limits.

Depending on your installation, a surge protection device may be interesting (e. g. https://www.clamper.com.br/ is a local manufacturer), and it is cheaper that stabilizers.

Best option would really be an UPS (no-break). https://www.engetron.com.br and 
https://www.sms.com.br are examples of decent quality local brands.