Either get a profile editor like saweetnesstrev said (nHancer, YAPE, etc), or manually delete the settings associted with hl2.exe in your registry. Go into your registry under this key:
CODE
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Video\{F5857C38-087A-4CD1-98DD-D4DF85D1A681}\0000
The GUID on yours will be different, but generally it's the furthest one down the list. The 0000 is likely the same (first adapter under that id).
Now delete any entries starting with:
CODE
_hl2.exe:
Even though entries will appear in your NVIDIA panel, the values associated with them are gone and you can either forget about it or set defaults or redo your profile.
That's it! HL2/CSS works again!
Or
Download a program name nHancer and delete the halflife2 profile.. IT WORKS!!