When you try to install magento 1.3.x on Blacknight Unix hosting you may (immediatly) get the following error:

'Whoops, it looks like you have an invalid PHP version.'

Well, don't panic, this is a very similar situation to the one outlined in this earlier drupal post. Your hosting does have the correct version of PHP installed (5.2.x) it's just that the default version is an older one. You just need to disable the older PHP version, wait a few minutes and things should be ok then.

To do this, log-in to the Blacknight control panel, navigate to the 'Web Space' screen and then to the 'WebSite Configuration' screen. Hit the 'Edit' button at the bottom right of this screen and then make sure that the 'PHP support:' option is unchecked and that the 'PHP5 support:' checkbox is checked. Hit the 'Submit' button and wait a few minutes.

Now try the Magento installation again and the error message should be gone....