Does this usage show on the 'windows task manager'?
On the 'windows task manager' 'processes' tab it will show you how much cpu usage is used by various processes running on your computer. Look for anything running other than 'system idle process' that has an unusually large number in the 'cpu' field.
spyware, that's my first guess. Lots of spyware. Possibly some trojans, backdoors, etc. If your machine is not behind a router, put it behind a router or it will get taken over even worse.
adaware spybot search and destroy hijack this
and run a few antivirus products, other than the one you are running now. At least 2 or 3 more.
You probably have some rootkits installed too, those are much harder to detect.
This is a classic symptom. Start menu has nothing to do with this stuff, it's just some shortcuts. taskbar, sometimes, if you let every program you installed put itself there, which if you're like most users you did.