[blub]

try deleting in \Steam\SteamApps\your@mail\ the half-life folder (you can delete counter-strike and day of defeat to) and run the steam and try to run to games

if that wont work you can try redownloading the whole half-life by deleting in \steam\SteamApps\
file half-life engine.gcf and half-life engine.gcf and entering steam and downloading it there