Which maintainance release of Mathcad 14 are you using?
In the release notes I find two entrys concerning problems with page preview:
Mathcad 14 M020
SPR 1425078: Mathcad exits prematurely when trying to Print Preview a non-empty worksheet.
Mathcad 15 F000
SPR 1779925: Fixes a problem with Print and Print Preview in specific situations involving the Microsoft Multilingual User Interface.
Maybe its the first one which you encounter (if you are using a pre M020 version). Updating to MC14 M020 should be free: http://www.ptc.com/support/mathcad_downloads.htm