I am assuming from your tags that these are Windows 2003 servers? If your experience is entirely Windows based, configuring Nagios in Linux might be a jump (I don't know the other open source monitoring systems listed by Kamil so not sure what OS they run on).
FAN (Fully Automated Nagios) might be a good option. Install it on some old hardware directly from an ISO. Has a couple of built-in popular GUI add-ons like Centreon and configuration scripts to get you started, including ones for checking disk space.
I'd second TheLQ as well, with seventy servers you need a monitoring tool, otherwise apart from disk space, how do you keep track of patches, anti-virus updates, cpu load, memory consumption and general hardware failures? Nagios can tie in easily with email alerts and (especially if your systems are quite homogeneous) you can probably get yourself to a place where you are alerted about the more common server problems as soon as they occur or even before.