Hi there

Got a quick question for you - is it a USB or a PS/2 mouse ? You can tell by following the chord on the mouse and pulling out the plug into the computer. If it is a round shape, then it is a PS/2 mouse - if it is a square/rectangle shape it is USB.
It will probably be to do with the power saving settings in Vistas interacting with the power management on your hardware (motherboard & BIOS). Now, if you don't know what the BIOS is - don't sweat on it! There is another fix that might be a whole lot easier.
I suspect that you have a PS/2 Mouse, as I have encountered this problem before with these systems. If it is a PS/2 mouse, then perhaps you can try a USB mouse as an alternative. They are relatively inexpensive and really easy to change.
Let us know what you think!
Cheers,
:)RG