True ^ And what do you mean reverifying?
Right click, properties, local files, verify. It did the trick - had to delete all other custom files then verify both portal and HL2. All solved!
See the title, basically.
I start the game up, and wait. And wait. And wait. And...yeah you get the idea. Nothing happens at all, you just get that lovely music.
Reverifying Portal now, just in case. Then I'll start on my HL2 files.