You know I had planned on playing through HL2 this week to get me back into the HL groove, but for some reason the game is stuttering something awful! It was fine up until mid-way through "Route Kanal" but I'm not at "Black Mesa East" and it's damn near unplayable. I've owned this game for something like 8 years and have had it installed on this machine at least once before without any trouble, so you can imagine it's bloody annoying.
I just hope it's just HL2 playing silly buggers and not the source engine suddenly deciding it doesn't like my machine. If so, the chances of be getting to play 'Black Mesa' this week are decidedly slim.
Try defragmenting your Steam files. That is the #1 cause of stuttering, due to the way the Source engine streams textures and such from disk. If they are spread out all over the drive, they will stutter if your disk isn't especially fast.