The SINPS315 software will run under Windows 98 according to the Siemens Hotline. You might be able to setup a Windows 95/98 virtual machine using VMWare, OpenBox or similar that runs within Windows XP. They have not tried it so it can't be confirmed if it will in fact work.
Also they wanted me to alert you that the CP315 is not longer available from Siemens and replacements are "non existent."
Be aware of a known issue communicating with certain older Sinumerik devices via serial port under Windows XP. The baud rate is limited to 1200, whereas it may go up to the maximum baud on the Sinumerik port under Windows 2000 or earlier. This cannot be explained, but may have something to do with the Windows XP serial port driver. You might try installing a Belkin USB serial port or similar, which may replace portions of the Microsoft driver and correct the problem.