Pando, Sorry for the wrong info. My machine is running on 512 RAM. System resources also seems to look farely perfect. One more thing, when I tried to create a new service name using net8 assitent, it gave me Dr. Watson error on ORADIM.exe. is this anyway related with our problem??
Though later on I copied the oradim.exe from some other server and replaces in my server, and the problem was solved.
But still main problem (ORA-12500) persists.
Shailendra, till saturday night I was working fine on that server, then sysadmin moved that server from one server room to the other, and from that point of time this problem is coming.