the MSN issue might be down to MSN servers. I've hear somewhere that they've siphoned off some of their servers specifically for the MSNv8 beta testers. I'm not entirely sure how MSN file transfers work as i dont think at any time you have a direct connection to the person you are chatting to. (thus each byte of data in the file transfer will have to bounce of MSN servers 1st)

as for the other problem you are experiencing: when was the last time you thoroughly cleaned your system and checked for virus, trojans and spyware? These are the usual cause for net slowdown if there hasnt been a physical change to your system