higher mouse velocity in horizontal direction

[debianer]

Hello everybody

I turn to you with a little Problem I have with my Debian Squeez running in VirtualBox 4.0.8. To the facts:

Host: WinXP SP3
VBox: 4.0.8
Guest: Debian Squeez (Guest Additions installed)

The host PC has two physical monitors. Thus i set up VBox to also use two virtual monitors. Within my Debian guest i was able to setup this
two-monitor setting using the xrandr tool (each monitor at 1280x1024. The same resulution the physikal monitors have). By the way: It is Debian with the KDE 4 desktop.

But as soon as I tell xrandr to use the second monitor to expand the KDE desktop on it (resulution then is 2560x1024) the mouse cursor has different velocities
in vertical and in horizontal direction. It seems as if by multiplying the vertical screen resulution by 2 also the mouse velocity is multiplied by 2. So what can i do to solve this problem?
At this point I have not entered those settings in my xorg.conf file. All the settings i just did on the console with xrandr or with its graphical frontend krandr.

I hope i was able to explain to you what the problem is.

Thanks