Protecting your information from outside intruders
Virus checker
The first piece of software you buy should be a full-featured virus checker. Norton Antivirus, McAfee Viruscan, or some other program…all protect your computer against most, though not all, of the latest nasties. After installing your virus checker, go online and update the virus list (and the virus checker engine, if available). The virus list tells your program how to recognize each virus, and it’s absolutely essential to update the list at least weekly. Norton’s Antivirus program updates itself without any prompting, if you set it to, and possibly others do, also. Set your virus checker  to scan incoming e-mail automatically, and to run in the background while your computer is on, watching for virus activity in RAM. If possible, have it scan outgoing e-mail also, so you won’t inadvertently send a virus to your friends. 

Firewall
A firewall protects your computer from intrusion over a network, which for most of us, means the Internet. Windows XP includes a firewall program, but third-party programs like Zone Alarm or BlackIce may provide even more protection. If you are on a high-speed always-on Internet connection, a firewall program is nearly as important as a virus checker. Check for updates to the firewall program monthly. of Zone Alarm provides a free version which is pretty good, along with for-pay versions with even more features. If you have a home network with a router, chances are the router includes a hardware firewall.

Spyware detector
Spyware is a type of program that someone installs on your computer over the Internet, without asking you. Technically, it could be considered a virus, but it doesn’t do anything to harm your computer. It reports to whomever installed it where you go when you browse the Internet, valuable marketing information. I don’t know if that bothers you, but it annoys me greatly. So I installed Ad-aware, a free program that detects and removes spyware that may have gotten installed when you were browsing. Run it about once a month, or more often if you spend a whole lot of time online.

Spam filter
Spam is defined as unsolicited and unwelcome e-mail. By that definition, an unsolicited e-mail that interests you is not really spam. There are numerous spam filter programs, some provided by your ISP, and none of them work perfectly. My ISP, Earthlink, provides one called Spaminator, that catches 90% of incoming spam, but even so, I still get an occasional e-mails offering to make me rich, enhance my sexual performance, or sell me something. If your ISP doesn’t provide a spam filter, find one that works with your e-mail program. Spam filters usually collect e-mails that they suspect of being spam, so you can identify any that you want to read and delete the rest. Check your spam filter accumulation at least weekly.

File and disk maintenance
Disk Cleanup
One of the most valuable utilities Microsoft includes with Windows XP, Disk Cleanup removes files that clutter up your computer. It’s particularly good at removing files left in your Internet cache after you spend a lot of time surfing the net. After my wife indulged in a particularly energetic evening of Internet exploration, there were nearly 100 MB of left-over cache files on the hard drive. Disk Cleanup also removes standard temporary files that Windows creates while you’re working, as well as an assortment of other files that you may not need. I run Disk Cleanup after each day’s surfing the net, and usually wipe out around 10 MB of trash files.

Disk Defragmenter
Files on hard drives tend to get split apart into separate pieces as you work with them over a period of time. The more you use a file, the more pieces, or fragments, it’s split into. The operating system keeps track of all the pieces, so there’s no danger of losing the files, but the hard drive takes longer to read fragmented files because the read head jumps around the drive as it reads all the pieces. Moving all the fragments into a contiguous stream makes it easy for your drive to read the files. That’s what defragmenting does. Windows’ Disk Defragmenter, in the System Tools folder, does that for you. Normally, it’s sufficient to run this program monthly, but if you add and remove a lot of files, you may want to run it weekly. Windows XP’s disk defragmenter is somewhat puny; for a better version you’ll need something like Diskeeper 7.0.

Checkdisk (Scandisk for Windows 95, 98, or Me users)
This program checks all the information on your drives (files, indexes, folders, etc.) for errors, and attempts to fix those errors. It’s especially valuable if you start to see disk error messages. Normally, run it monthly, unless you see lots of error messages. Windows 9x’s Scandisk is easier to use, since it runs from within Windows. Checkdisk is still used by Windows XP, and only runs when you boot up your computer. I hesitate to say it runs in DOS, since Windows XP doesn’t use any DOS code, but the effect is the same. 

Sources
Here are some sources for Programs that could be useful in keeping your computer healthy. I list these as references only, and don’t guarantee they will work with your computer or software. I haven’t even tried most of them.

Spam Filters
Mailshield  
SpamKiller 
SpamWeasel 
Spam Inspector 
Supergoogie 
iHateSpam 
Junk Spy 
MailWasher 
Matador 
SpamButcher 
SpamCatcher 
SpamNet 

Firewalls
Zone Alarm 
BlackIce 
Tiny Personal Firewall 
Norton Internet Security 
McAfee Firewall 
Outpost Personal Firewall 
PC Viper 

Antivirus
McAfee VirusScan 
Norton AntiVirus 
Panda 
PC-Cillin 2003 

Ad blockers
AD Killer 
Ad Blocker 
AdSubtract 
Pop-up blocker 

Spyware remover
Ad-Aware 
System Detective 
Nitrous Anti Spy 
OptOut 
Spy Ware Remover 

Disk Defragmenter
Diskeeper 
Norton SystemWorks 2003 

Disk Cleanup
CleanUp!