Go to windows update and make sure you OS is up to date. If that doesn't work try reinstalling steam, and downloading HL2 again.
Are you using a non NTFS partition, or any other non standard disks (sofware raid etc)?
Good points, however if I wanted to kill things easily in new fun ways... I would set the difficulty to easy. A difficulty setting of hard, should be just that, hard!
I have not completed HL2 yet. I believe I am about two thirds through now, having formed up with my first squad about 20 mins ago. So far I haven't come up against anything that has been too hard. The only spot I have found remotely difficult was setting up the first set of turrets.
Has...
If you buy it on Steam now you can play it as soon as you have downloaded it. It might take you a while, and shouldn't be attempted on 56K uless you have a lot of time on your hands.
Oh... anf overclock you graphics card. I have mine running at 410MHz. Take a look at http://www.guru3d.com/ (and other similar sites) to make sure you have everything running as well as possible.
You should be able to... make sure you have the most up to date BIOS and graphics card drivers. Update windows, make sure you have no IRQ conflicts etc etc etc... and you will be fine.
The 'in' value, as you rightly say, is the rate your computer is receiving data form the server, in kb/s. If this rate gets close to the maximum rate your Internet connection can support, it shows your cl_updaterate setting might be too high, as could your cl_rate setting.
I have played...