The first line of defense is to make sure the Windows operating system has the appropriate and current Microsoft Security and Critical Updates. Next is a good antivirus program and an additional (and preferably independant) anti-malware program) which will be close to worthless unless you are (or it is automatically) downloading and installing the latest virus definition files. Java, Adobe Flash and Reader should also be current. These are common ways for a computer to be compromised. We often find computers with some of the best software available to combat these problems, but because the software was not updated or used on a regular basis, the PC runs slow, or is infected.
Now that all software is up to date, the next consideration is a firewall. There are two kinds, software firewalls like the one built into Windows and hardware firewalls like those built into Gateway Routers. The Windows XP (software) firewall suffers from two major flaws, it does not block outbound traffic, and it can be switched off by another application, possibly even by a clever worm. Vista and Windows 7 offer more protection as do third party programs, but the definitive solution is hardware with stateful packet inspection (SPI) combined with network address translation (NAT) - which almost all name brand routers employ.
If you suspect a virus or other invasive program, you can scan your system online or with retail antivirus software. The web links listed in Online Help could be helpful in determining the problem or in confirming your system is clean, if you are in doubt. Please follow the manufacturers directions. Please note that not all malware can be reliably removed from an infected computer, particularly with multiple infections involving rootkits.
