UPDATE: A lot of this security is now built into Windows, so check your settings do protect you properly - if things are set up right, you are probably ok ... Personally I use Malwarebytes, Mailwasher Pro, and Windows own Defender - but other configurations are also safe. Just check and don't disable security provisions without being sure you've installed an equivalent of some sort.
But having said that, there is some excellent preventative software available, for free, that can help you. Get a good scanner, and scan any programmes you install on your computer. Have a good "background" scanner that will check if a virus becomes active in memory. Install a firewall to guard against any intrusion from outside, and to monitor any programmes on your computer sending messages about you that you are not aware of. And get rid of unwanted "spyware" which monitors your net habits, and sends information about those habits to commercial operations that sell such information to other companies.
The following software I find useful (under Windows) for these purposes. The first programs can protect you against common viruses and trojans, including email script-attachment viruses like "LoveBug" and its successors. (These are currently the most common form of attack.) Then I list two firewalls, designed to help you prevent unwanted access to and from your computer from and to the net. (ZA also has an email "quarantine" feature to block "viral" script attachments.) The last will help you remove "spyware" that may be sending information about your net habits to various companies. All these programmes are free for personal use. Commercial versions are also available, should you insist on spending money!
F-PROT - Use F-Prot to scan any new software you install on your
computer. The folks who make this programme (and others) also have an
information page. Check out warnings and news about viruses
before you act on them or worry about them. (Especially check out
emailed warnings to see if they are hoaxes. Don't send hoaxes to your
friends because you didn't check first.)
Update: This company no longer offers free software, and it has
discontinued its support for the DOS version I recommended. Pity!
Inoculate-IT - This is an excellent "background" anti-virus scanner.
Update:This product has ended its free version, and the company is instead selling a commercial version, eTrust EZ Antivirus. I am currently using AVG myself.
Other Anti-Virus Programs: I have also used Anti-Vir Personal Edition and AVG Free Edition, which seem to be excellent. I am currently using AVG.
In addition, here are two sites that list free antivirus software: The FreeSite.com and FreeByte. To see an independant ranking of this software, check out the Virus Bulletin 100% awards home page.For protection against viruses, trojans, bugs, etc, that may appear in your email, you can try the filter Benign which eliminates them before you read your mail. The same folks also provide a really excellent spam filter MailWasher Pro which I use all the time. It allows you to filter your email using a variety of criteria, and also comes with access to a large and growing database of known spammers. It works with regular ISP mail services, as well as net-based ones like Hotmail.
All these anti-virus programmes will detect all the common viruses and trojans, including Word and Excel macro viruses, email script-attachment viruses, and infected programmes. F-Prot is a command-line ("DOS-style") product, and so needs a bit of "personal intervention"; Inoculate-IT, Anti-Vir, and AVG are Windows programmes, and will run unattended. (F-Prot also has a background Windows version, but that is a commercial product. If you are willing to pay, however, it is probably the top-rated product of this sort available.)
Zone Alarm - This software firewall can guard your computer from
probing hackers and more importantly, will let you know what programs
are trying to act as servers (in other words, who is snooping on your
Update: Recent free versions of Zone Alarm regularly interfere with other software, and I have stopped using it. I cannot recommend it myself, although there are still some who favour it. I have also used Sygate, as listed next.
Sygate Personal Firewall - I had trouble with Zone Alarm, and soswitched to Sygate Personal Firewall - at the time free for personal use. If you prefer to try something other than Zone Alarm, I suggest this (at least I was happy with it!).
Update: Sygate has been taken over by Symantec - I haven't tried any of their new products, but continue to use the old Sygate firewall. I hope to try something new (and free?) sometime soon. (Update: I now use the built-in Windows firewall.) Meantime, many folks recommend Kerio (now Sunbelt) and Tiny Personal Firewall (now CA). With new security built into Windows (esp Vista), maybe this is something you can relax about - just be sure you have your system set up to give you maximum protection.
Ad-Aware - Do you have "spyware" snitching on you? This programme
can find any such programmes, and remove the snoopers.
Spybot - Search and Destroy is also very reliable, and free.
And here is an excellent site for further information (and to test your computer's defences!):
Steve Gibson's Shields Up Site
[ An excellent site for information and software, but be sure to try his "Shields Up" test to see how vulnerable your computer is. Just click on the "Proceed" link (once you're read what he has to say about other matters). ]
DSL Reports (another page that will test your ports, to see if you are vulnerable to outside "attack")
[ This site is currently not offering this service - check again later to see if they've re-instated it. ]
So: my recommendation is to download and install these programmes, read the manuals and help files that come with them to ensure you get the most benefit, and visit Steve's site for further information. Keep your software up-to-date - especially the anti-virus programs (regularly and frequently update their definition databases: they all allow this to be done automatically). And, of course, think about what you receive in your email - don't open attachments without being confident they were really intended for you.