Keeping your Windows up to date is very important, as it will install patches to fix any security vulnerabilities in the O/S. Updates are much simpler to do now, than they were years ago, especially if you have broadband Internet. It is as easy as making sure that Automatic Updates is enabled, and Windows will download and install updates in the background as you work. Only rarely do any of these updates require you to reboot your computer, and they will create a backup restore point before they install, so if there is a problem, you can revert to a known "good" configuration and uninstall the update without effecting any of your data. If you have a dial up connection, just go to http://update.microsoft.com to manually install them. This process is also simplified, allowing you to select to install only priority updates, which are automatically detected. Windows Vista has a ton of built in security measures, including Firewall, Anti-Spyware scanning, etc., which will take out 99% of the guesswork in configuring your "security system".
If you have uninstalled your antivirus (AV) software because its subscription ran out, you need to get a current version. Many of the major brands only cost about $30.00 for a one year subscription, or there are several free antivirus programs available from reputable companies, or ask your ISP (Internet Service Provider) if they provide AV protection with your plan. Many of them do now. It is crucial to have AV software installed, running, and up-to-date, because as I said... there are new viruses coming out all the time, and they not only self-replicate, but adapt. A good AV software program will download new virus definitions on a daily basis and will do a better job of blocking them.