I am far from being a techie but I'll give it a shot.
First off, what kind of internet connection do you have? Secondly, did you just recently install yahoo messenger? Sometimes vmk does not like new programs being added after you started vmk. I've had Yahoo IM for as long as I can remember and never had problems between the two. So adding a new big program can affect how your vmk runs. Be sure to clear your cookies and cache regularly and not have big programs running in the background. Also, if you installed Yahoo IM recently, did you accept the toolbar that came along with it? If so, uninstall that toolbar, it's nothing but spyware in my opinion.
Do all of your regular clean ups to keep your pc running in tip top shape: defrag, disk clean up, run your virus and ad-aware programs regularly. Another thing I highly recommend is using Mozilla as a browser, especially with the new release. It has built in spell checker for boards, blogs and other places you may need it. Comes in handy for me since I can't spell to save my life. lol
So mainly, look for any big program you've added recently. Uninstall it and if that gets vmk running better; then that was your problem.