It's getting more and more categorisation nowadays.

Some are called worm, trojan, rootkit etc - but to generalise it, I'd still call it virus

Few things I can summarise - in order to prevent those malicious program from infecting your PC

1) Don't double click the pendrive drive from "My Computer", after you get your drive back from your colleague - this will autorun the virus in the pendrive, and start infecting your PC

Instead, you should right click on the drive and choose "Explore"in "My Computer"

2) Don't use Symantec Antivirus - it's the worst (but very popular) antivirus you can get.

I've been using Kaspersky for 2 years, and am still happy about it.

3) Use better internet browser - instead of Internet Explorer

I'd highly recommend Firefox 3, which I've been using for the past 4 years.

Better still, the last but the best solution - don't use computer