You may not care enough about bug fixes and minor features to keep every single app on your hard drive current, but you'll need to keep Windows and a few major apps (such as your browser, your PDF reader, and your office suite) updated to avoid nasty security exploits. Fortunately, you can arrange to have all of these updated automatically.
Updating Windows. Open Windows Update in the Control Panel. Click Change settings, and set the drop-down menu to Install updates automatically. If you don't have Microsoft Update installed, only Windows updates will download automatically. If you have other Microsoft apps installed (anything from Silverlight to Office) you'll need Microsoft Update. To pick it up, click the Click here for details link at the bottom of the Windows Update window.
Updating Your Browsers. Every major browser has an automatic updating function of some sort, so your browser should already be covered. Microsoft delivers Internet Explorer updates via Windows Update, and Google Chrome receives its updates behind the scenes; to see whether your version of Chrome is current, click the wrench icon and choose About Google Chrome; if your version is old, the dialog box should give you the option to update.
Like Chrome, Firefox handles its updates internally. Get the latest releases from Mozilla by opening Tools, Options, Advanced, Update, and checking Automatically download and install. Safari's updates are handled via Apple Software Update, which normally is installed with Safari. If it doesn't run automatically, open it and go to Edit, Preferences, Schedule, where you can select your preferred update frequency.
Updating Adobe Acrobot. Definitely keep Adobe Acrobat updated because it's a popular target for malware. You can set it to update automatically by selecting Edit, Preferences, Updater and checking Automatically update and install.
At this point, all of your most critical apps are set to update automatically. But why stop there? Get the FileHippo.com Update Checker, a free app that will scan all the apps on your PC, check their versions against its database, and point you to download links for all the apps that need updating.