Is this a new problem? Were things working OK previously? Any new hardware or software?
If this is a new problem try a System Restore to a point prior to the problem:
start > All Programs > Accessories > System Tools > System Restore
If that doesn't help then:
As a first response you might want to post a HijackThis log for review by our Pros in that section:
Please follow the instructions
HERE & then post your log in a new thread in the
Spyware, Adware, Viruses and HijackThis Logs section.
(Not in this section please).
Please specify what issues you appear to be experiencing along with your log.