I am having a similiar problem to Roadrunner. If I turn sharing off it seems to go back to normal speed, but with sharing on it stutters, and any action seems to take a long time. It has crashed (frozen) on me quite a few times and as Roadrunner says the only option is to close and restart.
I am using XP pro.
The only time I have had this problem in the past, is when my Internet connection has been on the blink. I live a fair distance from the telephone exchange, and when the connection starts to drop out, then the problem occurs.
To test this, disconnect your PC from the internet, run RB and deselect "share data" and see if the problem still occurs. Do run RB for an hour or so to see the results.