System Utilities
A lot of the utility software you used to buy separately -- defragmenters, memory managers, disk cleaners and so on -- is either bundled into Windows or no longer really necessary. There are two major exceptions: the first is anti-virus software, and the second is a firewall.
In the anti-virus category there's a high-quality free product in the shape of AVG from Grisoft. It is regularly updated, and the company makes the effort to issue patches and tools to remove the latest new viruses as they emerge. The only price you'll pay for using AVG as opposed to paying an annual subscription is a little bit of time and effort every month, as you'll need to update the databases manually. If you're willing to shell out US$39.95 (~AU$60) for AVG's professional version, you can make the updates automatic.
The most popular free firewall software is Zone Alarm, which monitors your Internet connection and allows you to block any undesirable activity. For the first few days, Zone Alarm can be annoying, since every time an application attempts an outbound or inbound Internet connection a dialogue box pops up to ask you whether that application should be allowed access. You allow or block the application once, and Zone Alarm remembers the setting. After a few days, the only time you should be interrupted with a query is if you install a new application, or if someone attempts an unauthorised operation.
Zone Alarm isn't perfect, but it's better than nothing and many people like it. If you have a home network, however, you may find it troublesome; in such a case your cheapest option is probably a router that includes a firewall as well as multiple ports and other functions.
