As any IT veteran will tell you, Windows is fairly lean on utility software - which is ironic given how much tweaking it needs in order to run as smoothly as possible. Here are some useful programs which will make your Windows-using life a lot easier.
Spyware and malware have become some of the worst problems facing Internet users in recent times (for a basic guide to malware click here). Ad-Aware is a powerful spyware remover which will sort out almost any unpleasant software you throw at it.
These days, an up-to-date virus scanner is essential, especially if you have any kind of Internet connection. AVG Free Edition is a powerful and complete free virus scanner.
AVG Website (free registration required)
This program optimizes your computer's memory and cache management.
It boosts overall PC speed slightly, especially when running many applications at once. It also vastly improves stability under Windows 98/ME.
This is a handy utility for batch scripting which allows you to manipulate GUI windows from the command line, including hiding them.
This program will tell you which process is using a file.
Handy for when Windows insists that a file is in use and cannot be deleted.
This is an absolutely fantastic RAM tester for x86 (Pentium-compatible) systems.
It comes as a bootable .iso CD image; just burn then boot from the CD, and it will begin testing your RAM.
It is very comprehensive, with several powerful testing patterns.
This very useful utility adds an extra applet to your control panel that lets you manage all of your startup programs.
A simple little utility from Microsoft that allows you to make some changes to the Windows interface.
TweakUI downloads: Windows XP, Windows 2000/98/ME
This brilliant program creates a visual treemap of your hard disc, showing all the files on it in relation to each other's sizes. Brilliant for when you need to work out why you've only got 50 megs left on your system drive.
Without a decent firewall, your computer is susceptible to attack from viruses or worms and dodgy software has free access to the internet.
ZoneAlarm is simple to use yet secure and safe, protecting you from malware and controlling which applications are allowed access to the internet.