Chris (if that is your name, its the steam account tho)
Failure to lock down a vertex buffer is a graphical error, so first stop is dxdiag, run the tests in the graphics section.
(type dxdiag in the run box of the start menu to get the program up, everyone has it, as it's the directX diagnostic.)
If hl2 (and mods) is the only game to crash, uninstall and reinstall it, otherwise move onto the next step:
if not, uninstall gfx drivers and reinstall latest version and also make sure DirectX is upto date (9.0c is the current version I think, last update was feb07, so make sure you've got it:
Linky)
If that doesn't, try running it in other format, such as software or opengl (if you can, I cannot remember gfx settings for the source engine, can't be arsed to check)
Then uninstall/reinstall steam and hl2 if no more joy.
I'm gonna presume that the computer works fine for other games and doesn't crash out, so if all of the above fails, contact support at valve.
@Lee, nah, x-fire is just picking up the error, it's referencing the hl2.exe and the hl2.exe has also thrown up an error (look at task bar), I'm pretty sure xfire is just trying to explain the error.