The most popular (free) antivirus program around here seems to be AVG. Avira is also frequently recommended.
The most popular (free) firewall is probably Zone Alarm Free edition. Comodo also has a lot of fans.
Many people would recommend having both Spybot Search & Destroy, as well as Adaware, on any PC. (They're both free anti-malware programs). Malwarebytes is another good program in this area (and possibly the best).
IrfanView is the best free picture viewing program, which is also good for basic image manipulation. For the nearest free equivalent to Photoshop, use GIMP. There are also lots of free goodies here:
http://www.freeserifsoftware.com/
Windows Media Player is reasonably good at playing and organising audio files but there are two free video players which far outclass it. I recommend having both VLC and GOM on your PC.
Audacity is by far the most popular (free) audio editing program.
TheSage is a (free) really handy little dictionary.
Free&Easy Font Viewer is a lovely little utility which lets you quickly choose an appropriate font for any application.
Firefox is a vastly superior web browser to Internet Explorer.
Chris