I don't sit at a Windows workstation, so I never do this, so I know
nothing, but ...

> i am using Hp-ux=11i v3 on Hp-90xxxx server, now i want to export
> display to my workstation which is on XP-os,

   How, exactly, are you connecting to the HP-UX system?  Some kind of
SSH client/terminal emulator?  (Which?  Connection details?)

> i am also using Xmanager to get display to my workstation/pc, [...]

   As usual, showing actual commands with their actual output can be
more helpful than vague descriptions or interpretations.

> "/usr/bin/X11/xterm Xt error: Can't open display:localhost:10.0"

   Is there an X server running on your Windows system?  (Which?)

> but i am unable to set display variable from the following command
> export DISPLAY=localhost:0.0
> xclock

   I assume that you really can set the DISPLAY variable, but that doing
so does not give you the results you want.

> >I don't think "localhost" is correct?
> It is for me.

   It depends.  If your SSH software is tunneling the X traffic, then
the SSH server may have created a "localhost:10.0" pseudo-display, which
will get tunneled to the SSH client, which will use the real X display
there.  In that case, DISPLAY should be set automatically, and the user
should not need to play with it.

   If you don't wish to use the SSH X-tunneling scheme. then DISPLAY
should probably be set to something like "Windows_system:0.0".