As Smoerble pointed out, IT security is extremely complex and I would add to that: it's a process... it's an ongoing effort and not something that you can do as a one-off. Also, no matter what solution you pick, there is no guarantee that you will be 100% protected.
To answer your question it involves a lot more info about what you need, the size of your IT infrastructure and the budget you have. On the top, you have companies like Cisco and IBM the offer full solutions; as you can imagine you pay a premium... but they are very good. For the mid-range, I recommend Barracuda.
One of the best fits I see for your need is
Clear Concepts; the offer remote scanning for many types of vulnerabilities: Document Network Security Policies, Full network security scan, Document Security service pack deficiencies on the network, Identify installed applications with known security issues, Review IP communications setup - DNS - DHCP, Review Anti-Virus solutions setup & configuration (Document confirmed infections), Analyze Event Logs for reported system, Perform a System Registry scan and document on broken registry links and more.
Software wise, there is a huge range available from free to tens of thousands; without knowing exactly what you're after it's very hard to recommend anything. This
LINK has listed some software including in-the-cloud continuous monitoring; in this
ARTICLE, you can find the top 5 or so free software.
I hope this helps.