not sure sorry, is this buzzing more like a grinding sound? if you can open up the case and try to pinpoint where the noise is comming from it may help a bit.
how much ram do you have and how many programs do you have running/starting with windows? how much free disk space do you have?
try running something like cleanup click here then run a defrag. if that doesnt improve things try running a hard drive diagnostic just to check the hard drive. the online or desktop versions click here work with both my seagate and maxtor drives so it should work with most brands. if that all works ok post back.