My computer is running so slow and sometimes is not responding.

I’m having trouble doing the task, it says no hard drive found.

I have pop-up blocker that shows it is enabled, but I still get pop-ups every time I log. Also, my home page is set to Yahoo but it has been resetting itself to a "messenger site." I have to reset each time back to yahoo on a daily basis. What is this and what do I do?

When I run disk clean up in window xp; what is folder compress old file. Can I clean up my computer by deleting these files. will this impact on any of the existing working in any of the program

I want to make it so my computer is like the day I bought it. So basically wipe the hard drive clean and stuff like that. How do I go about doing that?