Maintain computer

I run a small charity developing an online system for providing yoga therapy for low back pain with herniated ('slipped') disc (yogatherapy.org). We have recently lost the person who managed our computers and need to find a replacement. All our work is done on PC's and the first task would be to ensure that our recently purchased PC has optimal settings and security arrangements, and access to some specialist programmes.
This is a small job and will be intermittent but it may increase in volume as our service builds up. Reliability and security are very important. We would like to build up a long-term collaboration.