You should never have two anti-virus programs running simultaneously on your PC, as this can lead to system conflicts. Therefore, you'll need to uninstall McAfee. Instructions are here:
http://ts.mcafeehelp.com/faq3.asp?docid=71525
While everything I've heard about Avast has been positive, I've no personal experience of it. Like the vast majority of the 'regulars', here on AB's Technology section, I use (and can thoroughly recommend) AVG Free Edition. It consistently scores 100% in independent tests of its effectiveness and it automatically updates itself daily:
http://free.grisoft.com/doc/avg-anti-virus-fre e/lng/us/tpl/v5
The most popular firewall, to partner AVG, is Zone Alarm Free Edition:
http://www.zonealarm.com/store/content/company /products/znalm/freeDownload.jsp
(Use the top download link. The lower one takes you to time-limited trial software).
If you need an anti-spyware program, Ad-Aware SE Personal and Spybot Search & Destroy both have their fans but AVG's own free product is also gaining in popularity:
http://www.lavasoft.de/software/adaware/
http://www.spybot.info/en/index.html
http://free.grisoft.com/doc/avg-anti-spyware-f ree/lng/us/tpl/v5
Chris