Sorry to hear you're having problems there.
We are having troubles with .tar.gz downloads with some users environments, we're still trying to find out why that happens. There is an alternative zip download for SilverStripe 3.0.2 here: [url]http://www.silverstripe.org/assets/releases/SilverStripe-cms-v3.0.2.zip[/url] which should work in the meantime.
The mod_rewrite issue could be resolved by checking and fixing the following:
mod_rewrite module for Apache is loaded. From my memory, there is a setting in the GUI for it in the WAMPServer system tray, alternatively, uncomment "LoadModule rewrite_module modules/mod_rewrite.so" in the Apache configuration file by removing the # before it.
AllowOverride All set in the Apache configuration. Search for "AllowOverride none" and replace those with "AllowOverride All".
The Apache configuration is typically found in apache/conf/httpd.conf, wherever WAMPServer is installed. Once you've made changes to the configuration file, you'll need to restart WAMPServer for the changes to make effect.
The next thing to try is using 127.0.0.1 for the MySQL server username in the SilverStripe installer, instead of "localhost". There are sometimes issues where using localhost takes much longer to connect to the database on Windows. Although I don't know if that's fixed in later versions, but worth a try.
Let me know how you get on.