Is there a downloadable, ready to use, virtual machine with sharepoint 2010 and all prerequisites already installed?
I just googled it but apparently it does not exist.
I just want a quick way to start making some tests.
Sure it is! Here is the link: http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=27417 It's huge because it has Project server, etc...
Depending on your needs, you may want to consider a trial hosted solution. I've recommended this to users and developers who want to test the SharePoint world with little upfront cost.
Andrew Connell has setup one here in a hosted environment. The VM is free and it looks like you can get a 14 days trial subscription to the service that hosts the VM.