ModelSim License Problem

I am using xilinx Tool Ver 6.3.
And Modelsim 5.8.

Due to some reasons I uninstalled modelsim.
After I installed it again I faced a problem related to license .
ModelSim is installed. and  While running it asks for license.

At licensing Wizard of ModelSim,
I choose a license file which was there previously( we kept a copy of
license file before uninstalling).

but gives following error msg:
---------------------------------------------------------------
Driver/Hostid is correctly configured.

License HostID detection report:

ERROR : The hostid type -Disk Serial Number- was referenced in the
license file
but does not match the value found on this machine.
  -the license file value for Disk Serial Number is:  e0ee2059
  -the actual value for Disk Serial Number is:  200e5645
---------------- and -----------------
XE Starter simulator license (xe-starter)
The hostid of the license does not match the hostid for this machine.
One of the following is likely:
   -The license is intended for another machine.
   -A dongle is not plugged into this machine.
   -The dongle driver is not installed or is not functioning properly.
   -The hostid mechanism has been changed or removed from this machine.
---------------------------------------------------------------

So I go for submit license request where they ask for Disk serial no.
There is a warning that one mistake in serial no. cost u $395 for
re-licensing.
I didn't know what is disk serial no. for Full VHDL.

I also have a latest copy of ModelSim where also i have to go for
licensing Wizard.

What is the way to come out from this?

"Is there any other Simulation tool which is free downloadable and
compatible with Xilinx 6.3 Tool ?"

Regards,
Sonali