A virus cannot release it's payload until it is downloaded/opened. The best thing to do is scan files for viruses before you open them. There's plenty of virus scanning software available on the internet (try
www.download.com and type in 'virus scanning software' in the search engine). There are also plenty of firewalls out there, some of which are free (the most common free firewall is called ZoneAlarm) while others you have to pay for. I use a firewall called BlackICE Defender, which fu(king rocks! Search for firewalls on download.com also - if you browse the web a lot, then you need a firewall.
Glimmer