This is a list of software that I install to computers that I build.

Security
Microsoft Security Essentials / Windows Defender
Free for unlimited home use or for small businesses on up to 10 devices. 

MalwareBytes Anti-Malware
This should be used alongside Security Essentials. Get the PRO version and use the “Real-Time Protection.” Lifetime license keys for version 1.x can still be found and will work with version 2.

Eset NOD32
For larger enterprises. NOD32 is fast because it is written in optimized assembly code. Tip: search the web for bargain licenses.

Windows XP:
If you are still running Windows XP, I strongly recommend moving to Windows 7. Windows XP is over 10 years old now. The main reason to avoid XP is that it is insecure, both because it does not have advanced security features built-in and Microsoft is no longer releasing regular updates to the system.

Utilities

Classic Shell
Make Windows 8 look and work just like Windows 7

7-Zip
7-zip is a free and open-source alternative to software such as WinZip and WinRar.

Sumatara PDF
Quick alternative to Adobe Acrobat Reader, a product that continues to bloat up in size, mess with your system settings in annoying ways, and get slower.