I installed SilverStripe using the newest recommended method.

everything complete without errors, but when I clicked "Launch you web application now" I got:
PHP Fatal error: Permission problem gaining access to a temp folder. Please create a folder named silverstripe-cache in the base folder of the installation and ensure it has the correct permissions in C:\inetpub\wwwroot\silverstripe\sapphire\core\Core.php on line 230

So I created a folder: C:\inetpub\wwwroot\silverstripe\silverstripe-cache
when I refresh http://localhost/silverstripe/index.php
I get 295 PHP errors, all referring to line 398 of: C:\inetpub\wwwroot\silverstripe\sapphire\core\ManifestBuilder.php
and ending with :
PHP Fatal error: Cannot write manifest file! Check permissions of C:\inetpub\wwwroot\silverstripe/silverstripe-cache/manifest-main in C:\inetpub\wwwroot\silverstripe\sapphire\core\ManifestBuilder.php on line 105

FIXED:

I reset the permissions in the security tab

First tried using an elevated cmd prompt to un-set Read Only (just to make sure) - that had no effect
Second, I right clicked on the folder -> Properties -> Security Tab, and set all the permissions to Full Access for each user. At first I tried just IIS users, but that didn't work so I just gave them all access. Silverlight completed it's install and I'm good now.