Microsoft has release bulletin MS06-075 which addresses a local privilege escalation vulnerability affecting Windows XP SP2 and Windows Server 2003 in the client/server run-time subsystem (csrss) which is a required component of Windows (in other words, it is always running on all Windows machines).  Note, Vista and Windows Server 2003 SP1 are claimed not to be affected at this time, as is Windows 2000 SP4.

We rate this one as important.  If someone can get access to the system via other means (cracked password, etc.) this vulnerability allows that person to elevate their privileges to become administrator by running a specially crafted executable.

References:
KB926255
CVE-2006-5585