Everyone should use a virus checker. The best type is a continuous monitoring program such as Norton or McAfee. Once installed, be sure to enable automatic updates or check for updates manually on a regular basis. It is almost always easier (and less expensive) to protect against a virus than remove one.

Use a Firewall

Firewalls keeps those who want to hijack your computer from gaining access to your system. Hackers use programs to seek out computers at random. A firewall is crucial for anyone with highspeed internet service.

Never turn your computer off with the power switch until Windows has shut down.

Hard drive defects can be caused by the hard drive heads contacting the surface of the drive disc due to improper shutting down. A host of Windows problems can also be prevented. Whenever possible, recover from crashes by pressing the Ctrl + Alt + Delete keys at the same time. Press them again to reboot your computer.

Exception - when your computer is frozen and Ctrl +Alt + Del doesn't help.

Run Scandisk and Defragment at least once a month

This will keep your hard drive healthy and prevent crashes.

-Scandisk is designed to check the integrity of a computer hard disk drive to verify that there are no significant issues which could cause problems with your computer data. Files are stored on your hard drive in data groups called "clusters." Sometimes these clusters can become "cross linked" with other clusters belonging to other files, or they can simply become "lost" from the rest of its fellow clusters. When you run scandisk the utility saves the "lost" file fragments into new files that you can view called "check" files (*.chk). It also repairs cross-linked clusters by making a copy and pairing it to two separate families (the original and the cross linked one). Scandisk stabilizes your computer.

-Microsoft Defrag or Disk Defragmenter is a software utility designed to help order the data on the hard disk drive. The theory behind defrag is to place each program together instead of scattered throughout the hard disk drive allowing the read-write head to access the data faster on the hard disk drive. When a file is too large to store in a single location on a hard disk, it is stored on the disk in discontiguous (not adjacent) parts or fragments. This fragmentation is "invisible" to the user; however. The locations of the fragments are kept track of by the system. Over time, disk access time can be slowed by fragmentation since each fragmented file is likely to require multiple drive head repositionings and accesses. (There's nothing you can do to prevent fragmentation, by the way.) A disk defragmenter is a utility that rearranges your fragmented files and the free space on your computer so that files are stored in contiguous units and free space is consolidated in one contiguous block.

Invest in a UPS (uninterruptible power supply)

A UPS will keep your computer from crashing during power outages, and will also protect it from high and low voltage occurrences. If you opt for a surge protector over a UPS get a good one! This is not the place to skimp.

Back-up important files.

You can use floppies*, Zip drives, a second hard drive or CD's or DVD's. Back-up important documents when you create them and perform complete backups on a regular basis. Think about what it would be like if your computer wouldn't start up tomorrow. Have a disaster recovery plan.
*Floppy disks have a high rate of failure and should not be used to protect vital information.

Never unplug peripherals when the computer is powered up.

This can short out the connector socket or the motherboard.

Keep your computer and monitor ventilated.

Heat and moisture can harm your computer. Make sure that there is nothing covering the vents on your monitor and that your computer as sufficient air flow around it. Your system should have at least 4in of space to the rear, with no clutter, loose paper, etc., to block the flow of air into the computer.

Run Windows Update/Software Update.

Windows contains many security holes that could make your computer vulnerable to outside attack. Attacks can include deletion of files or programs to complete erasure of your hard drive.

Keep good records of your installed applications.

Trojan, Virus, and Worm Information

Viruses are computer programs with the sole purpose of destroying data on our computers. The virus may only destroy unimportant files, or it may erase all of your document files. A virus can cause an infected computer to do funny things on certain dates, as well as issue serious commands such as erasing from the registry file, thus disabling the operation and booting up of our computers.
Viruses are spread through executable files we either get from friends, download off the net, or install a floppy disk. A virus will often come disguised under the cloak of a Trojan, which is the carrier for the virus.

Worms operate differently. Worms keep replicating themselves, multiplying themselves over and over. Worms generally come through our email client, but people can also get infected if they accept a Trojan File which has as the payload a worm. If you receive a worm program through your email, and then execute it, this program sends the worm file out to everyone listed in your email address book. If you work in a major corporation, this could means hundreds of people, and so the multiplying continues.

Phishing

Fake emails and fraudulent websites designed to fool consumers into divulging personal information and financial data such as credit card numbers, social security numbers, etc. Ebay, Paypal and Citibank members have been targets of phishing.

Spyware tracks browsing habits and can report back to various advertising companies. Spyware can disable Windows Updates, and Anti-Virus Software. Spyware runs in the background continuously. Removal can be complicated as it can write to the registry, start-up folder and also re-add itself during the boot process. It is much easier to prevent than to remove.

-Only download from reputable sites