How to determine the frame rate of a pc?

Go to "advanced settings" on the page that lists the resolutions.

I found that Hz info on the win7 machine that has lower resolutions but not the XP one which has the 1600x1200 output

Install Powerstrip. It will tell you more than ye need to know.

I just powered up my old XP notebook. On the video settings page, I chose "advanced", then clicked on the "monitor" tab. It shows the refresh setting (60 Hz in my case).

Another way to see the monitor speeds is to use the menu on the monitor (some monitors, anyhow). It's possible that your saturation-or-monochrome issue is related to the memory limits of the video (card?) - sometimes there are just not enough memory bits available to have both highest resolution and 24 bits of color information for each pixel. If it's really 'black', though, that's more likely a gamma setting or (old monitors) analog gain and offsets of the R,G, and B , as well as the (CRT) "screen" brightness control. The gray-bars test pattern is required for making these adjustments, as well as some HV precautions and insulated adjustment tools.

I had tried dropping back to 16 bit colour but the same "1 bit" saturation effect on a test colour bar image. I also had another rummage around for video settings but could only find display settings of a x b and no Hz info

There /has/ to be a "screen refresh rate" setting somewhere. The rate has to be within the range a monitor can handle. This is particularly true for CRT displays, which can be damaged by an excessively high rate.

I right-clicked the Windows 7 desktop, and selected "Screen Resolution". On that screen, I clicked "Advanced settings". On the "Monitor" tab, you can select the screen refresh rate.

Win7 is ok,ignore previous, the problem is with XP , happens to be Pro version, not showing the Hz settings.

XP shows the Hz setting, too. On the screen resolutions page, click the advanced link and then select the monitor tab. It looks different than 7, but it is there.

Perhaps the screen is organized with an incompatible setup to your bar-generating program? Right-click on the program, and see if there's variant video settings available (there's some version-to-version variation in the video settings matrix, a type that has been outmoded gets ditched in favor of settings for a new high-res screen sometimes). In this scenario, the 'black' parts are addressing into uninitialized memory on the video card.