Hopefully the Texmaker developers will address this problem. Miktex installs in the Program files (x86) directory on a 64 bit machine. Texmaker defaults to the regular old Program files directory when looking for the executable dvi, pdf, ps viewers. You have to manually set up the correct paths.
Texmaker is pretty good so far...i would like to try Kile, but i refuse to install Linux just to test it out. If Texmaker impliments a couple more things, they would be formitable. Right now, no tex interface is perfect. Some one smarter than me should take the one or two outstanding features in each and develop a new one.