If both computers are windows, you can install it in both with the same license. I think with lightroom 5, you still need a different license if want to use it in Windows and Mac.
If you are moving from Mac to Windows and no longer using Mac, support should be able to help you on that.
You can install LR5 on two computers as allowed by the Adobe License Agreement:
2.4 Portable or Home Computer Use. The primary user of the Computer on which the Software is installed may install a second copy of the Software for his or her exclusive use on either a portable Computer or a Computer located at his or her home, provided the Software on the portable or home Computer is not used at the same time as the Software on the primary Computer.