Precautions which can save your computer  from virus infections:

Generally virus infections causes through pendrives or cd drives.When an infected cd or pendrive is inserted then the autorun feature installs the virus in your computer.

1)So the first step would be to disable the auto run feature.In windows xp it can be done by typing gpedit.msc>computer configuration>administrative template>click on system>on the right hand side you will find an option called autoplay.enable it for all drives.

2)second precaution is whenevr you put in a pendrive dont access it by double clicking it,bcoz whn u double click the virus program inside it gets executed.So always right click and then explore it and copy the required files.Avoid copying folders as they generally  contains viruses. Files are normally not infected.Or before copying the folder just check it by rtclick and explore that it doesnt contain any suspecious files or exe.

3)also uncheck your hidden folders option and hide protecting file system file.This will help you in seeing the hidden virus file in the pendrive or cd.You can get this option at tools>folderoptions>view.

  The command to unhide hidden system file is :

Attrib –s –h d:folderfilename.ext

Attrib –s –h d:*.* (this will unhide all the files in the d: drive)

4)also try to use antiviruses which has program control and os firewall features.This helps you in knowing when a new program tries to access your pc.I always prefer ZoneAlarm.It has all these features.

5)Now when on internet or LAN always try to avoid downloading or copying suspicious files or folder.Keep your network firewall feature at high level which helps unwanted access.Avoid visiting porn site.I know its irrestible but atleast be careful in downloading something from those sites.These sites are most virus or spyware prone.

Another solution is install linux in your system and you are free to visit any site without any threat to your system.Or you can also use a virtual machine to experiment with viruses.One like vmware.