If you want to minimize urgent visits from the Computer Doctor, you should regularly carry out some preventative maintenance. In most cases, monthly is quite sufficient.

The Four Simple Operations are-

Disk Cleanup
Clear up the clutter with Disk Cleanup.
Windows programs make a lot of temporary files, which can accumulate at an alarming rate. In particular, the folders \Windows\Temporary Internet Files\ and \Windows\temp\ can really build up.

Here's how to do it:

Click Start, point to Programs, point to Accessories, point to System Tools, and then click Disk Cleanup.
Click the drive you want to scan, and wait while the program calculates how much disk space is available for cleanup.
To delete obsolete files, click the Disk Cleanup tab, select the check boxes of the files that you want to remove, click OK, and then click Yes.

To remove Windows components, click the More Options tab. In the Windows Components area, click Clean Up.
To see what is included in any component, select the component, and then click Details. If you want to remove individual items within any component, clear the check boxes of the items you don't need.

To remove installed programs, click the More Options tab. In the Installed Programs area, click Clean Up. On the Install/Uninstall tab, click the program that you want to remove, and then click Add/Remove. Follow the instructions on your screen to remove the program. 

Scan Disk
Clean up your hard disk drive with Scandisk
Through normal use of your computer, your hard disk drive can become cluttered and a bit messy. And folders can become cross-linked, file names can contain invalid or unknown characters or become damaged, and file names can become disassociated with their files. Scandisk can fix these and other problems for you. 

Here's how to do it:

Click the Start button.
Point to Programs, point to Accessories, point to System Tools, and then click Scandisk.
Click the drive you want to scan, and choose either the Standard or Thorough test.
Click Start.

The Standard test checks the files and folders on the selected drives for errors.
The Thorough test will give you further options. It performs the Standard test plus checks your hard disk drive for physical damage. You can even specify to scan areas of your disk containing only data files, or just the areas with system files, or both.
Both Standard and Thorough have a set of Advanced options that help you to deal with lost file fragments, invalid files, and files that overlap the same disk space. You can also choose to keep a log file of what Scandisk finds.

 Defrag (Defragment)
Defragment your hard disk drive to keep it running efficiently.
Files and applications on your computer's hard disk drive are not always stored together as one unit, but are often divided into smaller units and scattered around the hard disk. This is caused by normal use.

On a fragmented hard disk, a computer has to work harder to gather all of the small units to perform the commands you give it; this slows down its ability to access files and run applications. Running Defrag helps by putting all like files and applications into contiguous groups so the computer can access them quickly.

It's  a good idea to run Disk Defragmenter after you install new software to enable it to run at top speed.

Here's how to do it:

Click Start, point to Programs, point to Accessories, point to System Tools, and then click Disk Defragmenter.
Click the Settings tab, select the check boxes you want (to ensure fastest startup of programs, select the first box), and then click OK.
Click the drive you want to Defragment, click OK, and then click Yes.
Note: It is best to shut down all your applications before you run Disk Defragmenter.

Update Virus Definitions
Any virus scanner is only as effective as its most recent update, so obtaining frequent updates is critical to maintaining a secure computing environment.

Here's how to do it:The instructions below are for manual updating. Antivirus definitions can also be configured to update automatically.

Norton Antivirus:
Start Norton AntiVirus (NAV) or Symantec AntiVirus Client (SAC), and then click the Live Update icon. The Live Update dialog box appears.
Click Next. Live Update retrieves update information. This may take a few minutes. Click Next. Live Update begins downloading the updates. Depending on what you have selected and the speed of your Internet connection, this may take from a few minutes to one or two hours. You can cancel this operation by clicking Cancel. Canceling will not affect the current program.
Once the download is finished, Live Update automatically installs the updates and indicates when it has finished.

McAfee Antivirus:
Click on Start.
Point to Programs (XP users, click on All Programs)
Click on Mcafee.
Click on McAfee Shared Features
Choose Instant Updater
NOTE: Instant Updater can also be launched from with your McAfee product. Open your McAfee product and choose the update link from the Quick Jump menu.