You state that the free versions "haven't got sufficient cover". That's only partially true.
A good security system requires three main components. These are a decent firewall, anti-virus software and anti-malware software. The commercial products from Norton and McAfee (among others) are suites of programs, containing all three elements. You're completely correct in suggesting that AVG Free Edition won't replace the entire suite, since it only performs one of the three functions, but you can 'top it up' with other free software.
Windows (from XP onwards) has its own firewall but it's less than perfect. Many people prefer to use the free versions of either Zone Alarm or Comodo. (So that's one of the three security components taken care of, without spending a penny).
Next you need some anti-virus software. AVG Free Edition is the most popular but the free version of Avira also has plenty of fans. (So that's the second step taken care of, also free of charge).
Lastly you require some anti-adware/anti-malware software. You can choose between (or use all of) the free versions of Ad-Aware, SpyBot Search & Destroy and Malwarebytes.
You've then created the ecessary suite of programs without any outlay whatsoever.
If you insist upon buying a commercial product, avoid Norton and McAfee (which both have a reputation for being 'resource hungry', resulting in your computer running slowly). Kaspersky is much better.
Chris