Gordon Freeman runs way too slowly!

There are a couple of methods that might fix this.

1. First, try typing "sv_alternateticks 0" into the console (as linked by Shotgun Ninja above).
2. Try Alt-tabbing out of the game and back in. I've found this to be an issue with some Steam games, especially after opening the shift-tab dialog.
3. Change the shift-tab dialog's activation keys. This can be done by clicking Steam>Settings>In Game>Click on "Press Shortcut Keys" and switching it to something else.
4. And if nothing else works, check your HL2 key bindings. You can find them under Esc Menu>Options>Keyboard. The ones you want to look out for are those set for walk/run.