Themes blowing up install

I have installed WordPress twice today, and each time, when I install and activate a new theme (different one each time), the whole install goes down – I can’t even log into the admin panel. I can switch between the default and classic themes that are pre-installed and there’s no problem – this only happens when I try to activate another downloaded theme. (All the themes are being installed directly from the WordPress site, not third parties.)

Thanks in advance!

Francesca