I have something similar ... I just upgraded to 4.50 and now MSN is using 40-50% CPU on average in stead of the 0 - 5 % it use to use
basically slowing down my whole PC
when I uninstall it all together and restart MSNmessenger : flies like a hummingbird
when I reinstall 4.50 again : slows down again, CPU usage up to 40-50% again
no other changes to my PC in the last month