Android - Error / Unable to open class file R.java

Did a fresh install of Eclipse, JDK and android-sdk. I am currently receiving this error when creating a new project

[2010-09-26 16:07:56 - Test] ERROR: Unable to open class file C:\workspace\Test\gen\com\example\test\R.java: No such file or directory

What's the reason for this and how do I fix it?Eclipse Helios 32 bit java version "1.6.0_21" Android sdk API 8  

P.S. I'm new to Android development.

EDIT: I tried most of your solutions, but nothing worked. So I started using my friend's install of Eclipse Ganymede.

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