Mice trouble on Host XP >> Guest FC5 / Ubuntu

Hi all

I got a WinXP Pro running 2 virtual machines: Ubuntu and Fedora Core 5 (each with 30 gb assigned space)

Both VM's run well, even the network is functionning great (even though the XP has a USB Wireless external net card), however, from time to time, randomly, the mouse ceases to function in the VM's (keyboard works ok, though), and I have to reboot them, otherwise no way to make them work again.

Mouse is USB, keyboard is USB, RAM on the host machine is 2gb, HD is 300gb.

Any ideas?

Thanks