If it's laggy on start-up, that means you've either got a TON of start-up apps, you've got an infection, or like i7Baby said, you're overheating.
Try typing msconfig into your start menu search bar, and disable your start-up programs. See if this helps any, and let us know the outcome.\
My next guess would be a hard drive problem. Run command prompt, and type /chkdsk r
See if that helps, and let us know!