HQ and GSC Game World have been quick to issue the first patch for S.T.A.L.K.E.R., with patch documentation listing an impressive 68 bugs that have been addressed. Many of the fixes correct multiplayer issues.
Unfortunately, saved games from previous versions of S.T.A.L.K.E.R. won't work with the patch, which is a kick in the teeth. After installing the patch, you're instructed, go to C:\Documents and Settings\All Users\Documents\STALKER-SHOC\savedgames and "make sure any saves that were created prior to installing the patch are deleted. You will need to make sure that hidden folders are visible on the view tab of the folder options menu."