Or in the alternative, if it makes you choose, a program which explains clearly what is it that I would be erasing?

It’s an older desktop( Dell ) with Window’s 98 plus. I’ve deleted files, cookies. and history, but it runs slow and locks up a lot. Is there software to erase all data down to the operating system so as to start all over? Spyware Dr. says there is no virus. What would be a reasonable price to have it done? I’ve been told 0 by an IT.