Charlotte - I have found in this world that anything worth having is worth paying for. I have used Kaspersky AV for 7 years and without fail it has detected and procted me against things Panda, Norton, McAfee, and AVG have never even detected until much later updates.
Kaspersky AntiVirus is only free the first 30 days, after that it requires a licence. It is only about USD $ 50/£40/€40 per year, but in my estimation well worth the price. I refuse to work on any friend's computer unless they have Kaspersky installed because of my previous experience with the inferior performance of all other anti-virus software.
AVG is "good enough" to do if you cannot afford the cost of Kaspersky, but make certain to also install and regularly update
AdAaware,
Sbybot Search & Destroy, and
SpywareBlaster -- even if you install Kaspersky or any other anti-virus software.
You will also want to install a HOSTS file, this will automatically block access to many known bad URLs on the internet, this too will need updated regularly. I use hpguru's hosts file because it has never failed to block less than 99% of the worst of the worst URLs and is updated very frequently: get it
hereThe downside is that many "free" hosting situations used by simmers are known offenders that install all manner of horrors on your computer without your permission, with the HOSTS file you will no longer be able to access these sites, which really isn't that big of a loss considering the spyware and malware they will most likely install.
Let me stress again, NEVER open any attachment or file sent to you with an EXE on the end. Opening any EXE from an unknown person is the surest way to infect your computer, and even from known persons, since viruses do spread of their own volition via AIM, MSN, Yahoo, and email with regularity.