I have a web-server that has no accessible DVD drive, but have a license for SQL Server 2008 on disc...
Uploading the entire contents of the DVD is proving to be nigh-on impossible, so was wondering, if I downloaded and installed the trial version of SQL Server 2008 from Microsoft, could I then enter my license key, making the trial version into a fully licensed version?
Yes, you can do that. When you do the installation there will be a page to choose your edition and enter a product key. Should not be a problem for you.
Here is what the page looks like. http://www.kodyaz.com/photos/microsoft_sql_server_2008_rc0_installation/images/2660/425x318.aspx