Proper security on your computer and/or network is very important. Please review the steps and links below to ensure your system is protected.
1) Implement a hardware and/or software based firewall solution. A router is a hardware solution, and the best firewall solution, but software firewalls are also available. A full review and information guide can be seen at: http://www.firewallguide.com. Finally, Windows XP has a basic firewall which should be enabled if no other firewall is installed.
2) Install an anti-virus software product. Several programs can be purchased from Symantec, McAffee, Trend Micro, and others. Two FREE, fully functional, Anti-Virus programs are available for downloaded and installation on your system. Alwil Software provides Avast!: http://www.avast.com and Grisoft AVG Anti-Virus is another: http://free.grisoft.com.
(WARNING: DO NOT install more then one Anti-Virus program on a computer. Also, some viruses and worms can disable your anti-virus and firewall software without your knowledge. In these situations you will want to fully uninstall, and then reinstall anti-virus and firewall software.)
3) Update virus definitions on a daily basis and run a full virus scan on all systems at least weekly. Most Anti-Virus programs allow automatic updates and scheduled scans. Review the help file of the Anti-Virus program your using for more details.
4) Install all critical operating system updates and patches. Official Microsoft updates can be downloaded from the Microsoft Windows Update site: http://windowsupdate.microsoft.com/.
5) Install an Anti-Spyware and/or Anti-Trojan software solution. Again, several programs can be purchased from Symantec, McAffee, Trend Micro and others. Three FREE Anti-Spyware programs can be downloaded and installed on your system. Ad-Aware SE can be downloaded from: http://www.snapfiles.com/get/adaware.html and another, Spybot, can be downloaded from: http://www.snapfiles.com/get/spybot.html. For a limited time Microsoft is providing a free Anti-Spywear program which can be downloaded from: http://www.microsoft.com/athome/security/spyware/software/default.mspx
Microsoft provides good information on all of these subjects. You can view their recommendations at: http://www.microsoft.com/athome/security.