Some things I agree with
(or not as the case may be)
Monday, June 22 Problems with shaderapidx9.dll? This fix worked for me
I was doing some housekeeping on my PC earlier, and as it was taking up a fair amount of space on my C: partition I decided to backup my Steam install, uninstall it and reinstall to another HDD.
I backed up my installed games using Steam's Backup feature, then reinstalled Steam and restored my games (also downloading the newly released Smashball for good measure, which is an HL2 mod). Strangeness ensued however... Any time I tried to load HL2 or Smashball, I got a DX9 error relating to shaderapidx9.dll.
However, I stumbled upon a YouTube video seemingly offering a one-line fix... Here's my message to Steam support describing it all in more detail:
I'm running Windows XP Service Pack 3, fully patched and up to date. I uninstalled Steam earlier this evening so I could reinstall it to a different HDD - this went perfectly. Before uninstalling Steam, I also backed up my games using the Steam Backup facility. Once Steam was reinstalled on my secondary HDD, I restored the games. The only two games I had installed were HL2 and CS:s - after restoring those, I also downloaded Smashball.
My original Steam and games installation worked perfectly, however when I tried to load HL2 or Smashball (which is an HL2 mod), I got an error regarding shaderapidx9.dll and the game crashed out to the desktop. CS:S continued to load fine.
I did some searching around, and almost by chance I came across this youtube video: http://www.youtube.com/watch?v=lRcFfFxigpE. The video instructs you to open a DOS prompt and enter "net localgroup administrators networkservice /ADD" (without the inverted commas). I did this, and the command was issued successfully - I immediately tried to load HL2 and it worked perfectly, first time - the same for Smashball too.
Why should uninstalling and reinstalling Steam make this happen? It was working perfectly before, so why should a simple uninstall and reinstall cause this problem? Also, why don't you offer this advice as a potential fix on your Support pages? Cheers...
While I understand that there's quite a few Vista users out there who are having to muck about with the WMI service to get their Steam games running, this simple usergroup fix seems to have solved whatever the Steam uninstallation process broke. However, I'm not discounting that it was something the Steam platform update did (my client updated when it first connected, before I uninstalled - and I didn't play any games after updating the client, so it could've overwritten a DLL or modified something which persisted across the uninstall and reinstall).
That said, if you're using XPSP3 and suddenly any HL2-based game stops working, try inputting
net localgroup administrators networkservice /ADD
from a DOS prompt and seeing if your games start working.