This has affected Clamwin for at least the 2 years I've used it under WinXP Pro (no experience with other Win Versions).
When the Windows Taskbar is positioned at the top of the screen (as a proper OS should :-), the Clamwin "updates are available" box
pop up appears off screen in a position above the taskbar.
One may access the update window by switching to it with Alt+Tab, then Alt+Space, M (for Move) then arrow keys to bring it into position.
Once the box has been brought into the visible screen area it can be used normally to download the updates.
Presumably the position is determined relative to the taskbar, or to the position of the Clam Tray app
and does not take into account that entity's position on screen.