I wouldn't give you a nickel for all the system tools that come with Windows. That's because I can replace them with programs that do the job better without spending even that much.
Start with the firewall, which most people would consider an indispensable piece of software. Windows Defender's firewall is better than none at all, but not by much. Of the free alternatives, my favorite is the Comodo Firewall Pro. I described why and how I switched from ZoneAlarm to Comodo in a post from last February.
Since that time, I replaced the Comodo firewall and all my other free security apps with a commercial security suite. For me, the convenience of a single security program is worth paying for.
However, I recognize that many people will gladly put up with maintaining several individual apps if they can save a few dollars. For them, Comodo's a good firewall choice. Popular antivirus programs that are free for home use are AVG, Avira AntiVir, Malwarebytes Anti-Malware, ESET NOD32, and Avast.
Plenty of better browsers
Internet Explorer 7 has been a big improvement over IE 6, and early reports are that IE 8 will be a big step up from the current release. But Internet Explorer isn't even my fifth favorite browser, trailing (in no particular order) Firefox, Opera, SeaMonkey, Chrome, and Safari.
I acknowledge that some people have to use Internet Explorer--maybe their organization requires it--but the rest of us have no excuse for limiting ourselves to a single browser. The NoScript add-on (donationware) that lets you block scripts in Firefox is reason enough to use that browser. IE has nothing to compare with it.
One-step cleanup tool is the multitasking champ
I'm surprised that so many PC users don't know about Piriform's CCleaner (donationware), which does the job of about a half-dozen Windows applets. Along with a disk cleaner, you get a program uninstaller, a start-up manager, and a Registry checker.
You have to exercise a little restraint the first few times you run CCleaner, which empties your Recycle Bin, clears your Internet history, and performs other irreversible system chores. Still, I've been using CCleaner for several years and haven't had any problems with the program yet.
A new alternative for shoring up your drive's sectors
In a post from last March, I described the free Disk Defrag utility from Auslogics. My new favorite free disk defragger is another Piriform product, Defraggler (donationware). The program recovered 20GB of lost space on my laptop's 200GB hard drive, though in my unscientific tests it seemed to take longer to complete the defragmentation than it does when using Disk Defrag. This might indicate that Defraggler's doing a more thorough job, but maybe not.
You can also defrag from a command prompt. To open a command prompt in Vista, press the Windows key, type cmd, and press enter. In XP, click Start > Run, type cmd, and press Enter. The Vista Forums provide a detailed explanation of the many options you have when you defrag the DOS way.
Some people claim defragging does nothing to speed up your system. Even though my notebook wasn't necessarily low on disk space, I'll take that recovered 20GB any day.
Freebies for inveterate system tweakers only
Sysinternals, which is now part of Microsoft, offers a solid lineup of utilities for digging deep into Windows' darkest corners. Two of my favorites are Process Explorer and its cousin, Process Monitor. Once you get a handle on the information they present, the programs give you as complete a glimpse inside Windows--in real time--as you'll find anywhere.