Every Windows Based PC should have an up to date antivirus solution installed. A lot of the popular/advertised options have either a fixed or an annual cost. however, they may not always be the best and could even be resource hogs. There are a few free contenders out there that I am aware of here.

The rule of thumb is: Avoid Norton. It really is a system hog and replacing it with another alternative could give your PC a new lease of life.

Microsoft Security Essentials

Microsoft's Security Essentials is the new kid on the block and its free to all users who have a legitimate copy of windows, so its a good alternative to the others below where some have a "home use only" requirement.

It seems to work well, have decent detection rates, and since it is from Microsoft, it should have staying power atleast. However it may not be as performant or as fast as some of the others on the market.

Avast 4 home

avast! 4 Home Edition is a full-featured antivirus package designed exclusively for home users and non-commercial use. Both of these conditions should be met to be eligible use this product.

Avast has a resident shield, Email scanner, also heuristic email scanner where it may detect virii not current on it's database. It also has a few other scanning abilities, but a clunky default interface (which can be turned off) and it speaks to you. cool.

The major downside is that you need to re-register it on an annual basis, but also be aware that it is not as fast as a few of the later options.

AVG Free

AVG is another popular free antivirus solution that does active scanning, and scans emails. It is free for personal use and does not need to be re-registered on an annual basis.

AVG is fast, but there are suggestions that AVG may not be as good at detecting viruses as Avast.

Avira AntiVir

AntiVir is another free and lightweight antivirus solution. It does not have a built in mail scanner, so if you use some sort of email client (Outlook (Express), Thunderbird or other), this may not be suitable.

It is free for personal use, but has the annoyance of throwing up a full screen add every update (so once a day) advertising it's paid for version, where you need to click the ok button to close...

In my usage, AntiVir was even faster than AVG, but at the same time it was quirky on some installs - it simply refused to install on some PC's. Add to that the popup that it throws, I would prefer one of the other alternatives.

Kaspersky F-Secure

Not something that I have used, but Barclays has an offer for its customers to have F-Secure antivirus for free. This antivirus may not be as lightweight when it comes to resource usage as above programs, but it is full features and may have one of the best detection rates on the market.

ClamWin

ClamWin is a Free Antivirus for Microsoft Windows. It does not include an on-access real-time scanner. You need to manually scan a file in order to detect a virus or spyware. I have added this as it does not suffer from the "personal use" limitations of some of the above programs. Because it only scans for viruses I would not consider this as an ideal option for home use.

I have used AVGFree, AntiVir, Avast! and Security Essentials - all seems to be problem free - except for Avira, where there can be quirks when installing. However it is also the fastest one, so if it installs without issue, it may be the better option. After that, AVG talks to you and gives you a warm fuzzy glow. The other's also have their good points and there is plenty of choice it seems.

[this post is also available on my personal site.]