A computer just like a car, house, or other object can be cleansed, protected, cursed, etc. There are those who also have been able to send magic through the computer much like one can do through a phone or even letter. Magic is about energy manipulation and the connections it takes to manipulate that energy .
As with all magic one must also take the mundane steps along with the magical to achieve the desired end result. If your computer is running slower than it should try running some anti virus software, defragmenting your hard drive, and cleansing it with sage. After you can install an updated virus blocker and do a protection spell on your computer.