What is the best network monitoring and management tool

We are looking for a good network monitoring and management system that would give us notifications when a server or network is down, agent monitoring, and also allow unlocking AD accounts remotely. We had an issue 2 weeks ago with a virus that locked all AD accounts, and luckily we were able to get them unlocked by a soon to be former IT consulting firm. We would rather do it ourselves since we are establishing an internal IT department. Any suggestions for a product that is good and won't break the bank?