Possible Problems :
1. If it started recently then it is less likely to be a " wrong keyboard layout " selection problem.
2. Your keys are loose and get pressed automatically. If there is a calculator key on your keyboard it might be getting pressed automatically if loose. - Get new keyboard and dont spill liquids on it.
Test it by disabling the keyboard from Device Manager and using external Keybaord.
3. The least likely problem is having a virus/malware.