Before I start I want to say that I know what the "memory could not be read" error means.

I and playing the latest HL2 using Steam.  Whenever I get to a 'checkpoint' where it says loading, it will load, but then as soon as I start moving it will crash with the error.  I forgot to record the exact error last time, but if anyone thinks they could help, it aint much fun playing like that.

Thanks in advance.

This is a crash, as you might know.
And this can be caused by many different things. However, Valve has put a Support section on the Steam website where you can find more information about fixing the problem.
The current link below will send you to the proper fix.

http://support.steampowered.com/...