I select a large area of the screen to capture but the area actually captured is smaller than my selection. The actual image size captured varies around 1300 x 900 pixels.

This is on running PMView Pro 3.51 with Vista SP1.
My display resolution is 1920 x 1200.

Is there some subtle Vista or PMView limit at work here?

A possible factor: This PC is a laptop with a native resolution of 1400 x 1050 but displaying onto a larger external screen. The video card and driver are well capable of running this screen at the full 1920 x 1200. I have rebooted (a number of times) since starting to use this larger screen.

I note also that IrfanView exhibits similar behaviour, although the size captured seems to be slightly smaller.

So, do we have some Vista issue here?

This sounds like a setup issue. Is the notebook's screen disabled?

Windows (and hence PMView) limits capturing to the size of the desktop. You may have to configure the desktop size to fill all of your screen (in display settings). If the notebook screen is enabled, Vista may possibly limit what kind of configuration you're allowed to.

Take a good look in your Control Panel/Display Settings, and you might see what's going on.