1.Try to use Mozilla Firefox or opera or Apple safari as they have very few security vulnerability as compared to the Internet Explorer 7, which is full of well known security vulnerability. Although I have never tried Internet Explorer 8.
2.Never download or activate the grease monkey extension in Mozilla Firefox while orkuting because it may run malicious Java script without your permission leading to your private data export to the hacker.
3.Never click on malicious link or catchy photos from inside your Orkut windows as they lead to almost most of the Orkut hacks.