LSASS.exe, API error

When I turn on my computer it begins to load Windows and
it gets to the logon screen when the following message
appears.
(the title of the error) lsass.exe - system error
(message)
Insufficient system resources exist to complete the API
-my only option is to click OK and then the computer
restarts.
my computer had been working fine up until then but my
brother had turned off a program when he said the whole
thing froze up on him.
im running windows xp as an operating system