Here’s a list of companies that provide server administration services.
Most of these companies provide server administration for linux servers. But a few also support Windows servers. Check their websites for more information.
Normally, these companies will do security configuration and audits, software updates, server optimization and monitoring. And their mothly service will also include a few hours of server administration to fix any problems with your server software.
You can find cheap services, starting at $30 per month and per server. But a more reasonable price will be in the $50 / $100 range, for a more reliable service.
I can recommend Touch Support and Rack911.