Can someone be able to go on my old computer and check out the activity I had on it?
Unless you absolutely wipe the drive clean. And I mean way beyond a factory restore. People can find old information about your computer. if your getting rid of your computer yank the drives out and destroy them.
Ya, police etc can read the hard drive.
Yes once you do something on the computer it will never ever leave the memory.
Reformatting the hard drive would keep most users from being able to get your info.
Yes... You need to pull the hard drive and destroy it... A hammer works good