Ah the joys of being a system admin
All sorts of tools are available, but they generally rely on a level of sophistication and control not available to home networks. For example with Windows XP Pro (or 2000) on each workstation, coupled to a Windows 2003 (or 2000) server controlled active domain you can have all sorts of power over the PCs and users.
For the home network, presumably using windows XP the best you can do is assign restricted user rights to anyone you can't trust - but that then relies on you installing any programs. If you have a decent router connection to your internet the firewall on that will prevent most security intrusion attempts. Then there is just antivirus, make sure you have one that automatically updates and you shuould have most of the basics covered.
I tend not to worry about spyware, unless I start to see unexpected behaviour such as pop ups or misdirected internet home pages. You can get too paranoid over this stuff