DaveyD said:Edit: do what said above, then try this if it doesn't work:
Check that the game cache is validated, find your config file in steam\steamapps\username\half-life 2\hl2\cfg\ and delete it. Restart Steam and have another try.
DaveyD said:Is Half-Life 2 fully up to date and have you checked that it's 100% validated and defragmented? (Right click Half-Life 2, properties)