How can I fix my father's computer because i messed it up installing windows vista?

My father's computer is a 2005 model of a Dell Dimension B110 and It is designed for Windows XP Home Edition, I was so stupid that i decided to install Windows Vista Home Premium for him which turned out not very good. It kept giving me the blue screen of death like windows has been shutdown to prevent damage to your computer everytime when it shuts down on vista, So I reinstalled Windows XP back for him but this time the system is not working correctly, it is running slow and not functionning very fast even though it has 1.50 GB of Ram Memory. My father tried running his dvd copying software DVD Shrink and it took so long to process a DVD, it said 24 hours and 50 minutes remaining. Before when he used it it only took 30 minutes to process a DVD and burn it, it looks like the hard drive is messed up i think. How can I fix my father's computer ?? My father's computer is the main computer of the house because we use it to print stuffs and it's not working right, even the printing software is not working correctly. I putted the Windows XP Home Edition Disc back in and let it do a full format of the hard drive which takes two hours to complete, before i usually use the fast format option, will it restore my father's computer back to normal ??? Do I need to go buy a new hard drive ?? Can anyone please help me out ?? Please help me, this is the main computer of the house and have to get it working correctly again.