I think the problem may be in your strcmp, you have it like this:
<option value="<?php echo $row_RsCountry['CountryID']?>" <?php if (!(strcmp("<?php echo $row_RsCountry['CountryID']?>", (isset($_GET["invalid"])?ValidatedField("custregistration","country"):"") ))) {echo "selected=\"selected\"";} ?>><?php echo $row_RsCountry['CountryAbbrev']?></option>
I think this should be like this instead:
<option value="<?php echo $row_RsCountry['CountryID']?>" <?php if (!(strcmp("" . $row_RsCountry['CountryID'] ."", (isset($_GET['invalid'])?ValidatedField("custregistration","country"):"") ))) {echo "selected=\"selected\"";} ?>><?php echo $row_RsCountry['CountryAbbrev']?></option>
I removed the php block around the value in the strcmp and also add in some double quotes to concatenate the value. I also changes a couple of the double quotes around values to be single quotes. In php if you have a value that needs to be referenced in quotes that is not a string literal then you can using single quotes.