The easiest way in is through the front door.

It doesn't matter how well protected your network is, poor judgement and/or user error accounts for most malicious intrusions into your business environment.  Not everyone is a computer expert, nor should they have to be.  An errant 'click' here or a mis-typeed webpage there can lead to malware, spyware, and viruses establishing a foothold in your domain.  Good quality and well managed anti-virus and anti-malware products are a must for all computers in your network.  It is not enough to just install a program and hope for the best; it must be looked after and managed in order for it to do its job.

The only truly secure system is one that is powered off, cast in a block of concrete and sealed in a lead-lined room with armed guards.
— Gene Spafford