Already check if no have dns issue?

no DNS issue. i can ping to each other with proper reply even i used the bpclntcmd command to verify the DNS problem :(

i am able to take the flat files backups via virtual name successfully but not able to take the backup of SQL via virtual name......

On theother hand i am also not able to connect to the PC via virtual name if i tried to connect to the PC from RUN-->  \\virtualPC name.....but i can connect via \\virtual IP but still it asks me for password which i aspected not to ask bcuz the PC from where i am trying to connect to Virtual IP and the PC which is virtual IP machine both are using same domain user name

when i try to connect via virtual name i gets the below message

................................................
is there any service or other thing in windows2003 which faciliate virtual name???? which i should see !!!

Is this the Virtual Name of the cluster OR the resource name for sql ????

Can you post a screenshot from the Cluster Admin screen ??

sql-cluster is virtual name
i have already posted the screenshot b4 your post (this screenahot comes when i put virtual name in the START-->RUN)

what screenshot you are taking about from the Cluster Admin screen??

You need to be physically logged onto the active cluster node when you open the NetBackup for SQL Server interface.
You should be able to use Remote Desktop to connect to the Virtual hostname.
You need to log on using the same user id that is used to start the NetBackup Client server.
See the NetBackup for MS-SQL Admin Guide - Chapter 5: Using NetBackup for SQL Server

You need to be physically logged onto the active cluster node when you open the NetBackup for SQL Server interface.

i am able to login physically but i am useto login via terminal. Note:- my user account is Domain Admin

You should be able to use Remote Desktop to connect to the Virtual hostname

Yes i am able to connect remote desktop via virtual name

You need to log on using the same user id that is used to start the NetBackup Client server.

Yes i am using the same user account

Then I suggest that you log a call with Symantec. The engineer can connect via Webex and assist you with the setup.
There is clearly something that all of us are missing.
I have personally never experienced any problems backing up a SQL cluster (Microsoft and Veritas Cluster).

Have you read the section in the NetBackup for SQL admin guide under the topic "Support for Veritas Cluster Server (VCS) clusters"?