What is the context of your question? Do you want to know whether the person is usually in the same room with the PC or do you want to ask whether the person is near the PC at the moment? (During a support phone call, for example.)
Hi Tamara At the PC and in front of the PC are both OK and basically synonymous. That would be a position appropriate for using the PC.
Near or next to the PC are also OK. But in this case the person is probably not in a position to actually use the PC. To use the PC, the person would have to move to the at or in front of position.