insperion 1100 bad hard drive I need help

howdy im new to working on computers but want to learn I have a dell insperion 1100 and a 5500 and a few others im toying with my issues are with the hard drive. ok this one first 1100 its only got a 40 hd so i expect it to be slow but the hard drive toke a dump of some sort  now it loads to windows xp but to get the main page with your icons for internet trash and what not you have to go through task bar- new task-browse-then you can pick anything and open it and it brings up the main page. I want to restore it to factory settings. I dont have any disks and it wont let me restore. Im considering wipeing the hard drive and starting over but i dont know how or what to do ( exactly ) once its wiped what do i need to make it work again the computer has the codes for xp on the botom and its legal. it only has 30 % memory unsused and has mager glitches. I can get a disk to wipe it and thers nothing on it that matters to me i got it off the internet but i love learning about these. please help thoughts ideas welcome.