pageup/down keys behaving like ctrl+home / ctrl+end

Hi

pageup/down keys behaving like ctrl+home / ctrl+end.

Windows XP 3

reproduction example:
1) open xml editor that has hundreds or thousands of lines
2) press page-down key

Expected behaviour: cursor moves down down by a page
Current behaviour: cursor moves to end of document (as if ctrl+end was pressed).

Thank you and kind regards

Adam Żochowski