Yes, if you use ISE there is a problem if you try to use their
compxlib compiler tool.
Compxlib will only allow you to use approved/supported simulators and
ModelSim
Student Edition is not one of those simulators.
The only workarounds I know of are:
1) To compile the Unisim, Simprim and Xilinxcorelib source manually
into libraries
of the same names.
2) To download and install ModelSim PE from the
www.model.com
website. Since
no license is required to use the ModelSim PE compiler you can select
it from the
compxlib tool and allow PE to compile your Xilinx libraries.
I'm not an ISE user so I'm not sure if you will have problems
targeting ModelSim Student
Edition to simulate as well. If there is this limitation when you try
to launch your simulation
from ISE you can always create your design using ISE then export the
design to VHDL or
Verilog then compile and simulate the design using ModelSim Student
Edition in stand-alone-mode.