Free Essential Software

Thankfully, nowadays you can optimize and secure your computer for free. Most of the software links from "Essential" through "Utilities" come from download.com, where everything is tested for spyware. All work with Win 7, Vista, and XP, unless noted. When installing some of these, you'll be prompted to install a toolbar or similar; uncheck the box(es) if you don't want the extra stuff. Most of the file sizes listed are from June 2011.

A good monthly maintenance routine: Run CCleaner to remove junk files, scan your system with Malwarebytes and your anti-virus program, and optimize your disk drive.

CCleaner (2.9 MB) (removes temporary files, cached browser files, cookies, etc.)
"This program is a must-have weapon against system slowdowns, tracking cookies, and the multitudinous debris that can clutter your computer." CCleaner also includes a safe registry cleaner and other valuable tools.

Malwarebytes (9 MB) (removes spyware, adware, ransomware, trojans, keyloggers, etc.)
This program is also a must-have and doesn't interfere with your anti-virus program. Run it at least once per month with a quick scan. If it finds anything, run a full scan afterwards to make sure all traces of the malware are gone. For $25 (one-time fee), you can get the pro version, which starts with Windows and provides real-time protection, including website blocking.

Use only one anti-virus program: Avast (54 MB) · AVG (5 MB initially) · Microsoft Security Essentials (7.5+ MB)
If your internet service provider offers free security protection (Norton, for example), that's the way to go. Paid versions of anti-virus software are great, but why not get it for free? Avast, AVG, and MSE are well-known and very good alternatives.
If you have an anti-virus program installed, including a trial version, be sure to uninstall it before installing another one, or it can potentially cause conflicts that affect PC performance.

Auslogics Disk Defrag (4.5 MB) (for disk performance; takes files that are scattered all over and makes them uniform)
Windows 7 and Vista automatically defragment your disk drive if needed once per week at 1:00 Wednesday mornings, or the next time the computer is on and idle. Windows XP includes a basic defragmenter but it doesn't run automatically. If you would like to use a better alternative complete with graphical display and more control of the process, including file optimization, try Auslogics Disk Defrag. For most users, optimizing the hard drive is fine once per month, or anytime prior to installing (or after uninstalling) a large software program or suite. Once Auslogics is installed, it will disable Windows' defragmenter and its schedule. Tip: When running this program, click the drop-down arrow just to the right of the "Defrag" button and select "Defrag and Optimize"; this will not only defragment, but optimize your files as well.