Hello,
While your crop tool is selected, under your presets there are a lot of different size presets to pick from. I just did this playing with your question and change my preset to a 5x7, then I tired to change the numbers in the crop tool bar to 8x10 and it wouldn't allow me to do that, and it also changed my resolution of my image.
Since I do not know what setting you have selected, try mine!!

Do you see FREE FORM in your drop down menu for the crop tool? Pick that one, then in your tool bar I typed in Width: 8, Height: 10, Units: Inches, Resolution was there at 300.00/Inch, I didn't have either the Specify print size or the Maintain aspect Ratio checked. Then I click apply and it worked.
I have never had a problem with PSPX not cropping to the right size I stated, until I played with the presets!! There are all sizes of presets in there and the one for 8x10 works fine also, it just didn't work if I picked the 5x7 and then tried to adjust the size from there...
Let me know if this works for you. I have never read about the crop tool being broken in PSPX and something like this is usually a setting somewhere.
