No need to pay for Mcafee, in fact I would uninstall it completely.
The most comprehensive Viral, spyware adware coverage can be accomplished with freeware products alone.
Make sure you stay up to date on patches of viral and ad/spyware lists as well as any windows patches.
The following should provide excellent coverage when installed together as they seem to cover each others weaknesses.
AVG Free is awesome for Antivirus, make sure you automatically update, it outperforms MCAfee on most reviews.
Spybot Search and Destroy for Spy/Adware
After installing, I like the TeaTimer app with it because it prevents reg changes and Browser Hijacking. It can be annoying when installing or using SW that makes registry changes because it won't allow them without your permission so occasionally you have turn it off.
http://www.safer-networking.org/en/index.html - info
Download here:
http://www.download.com/Spybot-Search-Destroy/3000-8022-10122137.html?part=dl-spybot&subj=dl&tag=but
Microsoft Defender (info on their site)
Download here:
http://www.microsoft.com/downloads/details.aspx?FamilyId=435BFCE7-DA2B-4A6A-AFA4-F7F14E605A0D&displaylang=en
LAvasoft's Ad-Aware SE (Free version)
www.lavasoft.com - info only
Free Download here:
http://www.download.com/Ad-Aware-SE-Personal-Edition/3000-8022_4-10045910.html?part=dl-ad-aware&subj=dl&tag=top5
The only issue is now the number of processes running all the time so a beefier setup helps.
Enjoy