User Manager for domains

Hi,

When i clicks on User Manager for domains, menu of NT server, sometimes it gives me error regarding resources and at that time i am unable to create the user. Then i restarts my NT server and creates user.

Please suggest me what to do?
There is sufficient RAM on computer.
Service pack 6.0 is also installed and i have re-installed it also.
At present no. of users are 300 only.