Celtx exports as a text file (.txt), which you can open in virtually any word processor by right clicking on the file and going to "open with" (or something similar, it's been ages since I've used a PC). More advanced word processors will probably show the correct formatting.

Alternatively, you can format it and save it as a PDF (there's a tab at the bottom to do this) and then open it with Adobe Reader if you don't need to make changes outside of Celtx.