Well the way to check would be to watch your hard drive activity light, see if that comes on when you scroll.
Out of interest, how much RAM do you have? are you running a lot of other things at the same time as MSN?
If it is the hard drive working away then what could be happening is that your memory is busy full of information for other things, and so the computer is having to get what the information for what you're scrolling to from the hard drive rather than the quieter and faster memory. Might be worth a check for viruses or spyware etc that could be using it up.
This is of course unless someone has a better suggestion, as I'm a long way off being an expert!
PK!