why is it so laggy after i reformatted my computer? can someone help me?

hi..after of all i just bought a second hand computer for my friend with is pentium 3 window xp. After using for 1 month, i find that the com is getting slower! i suspect it due to virus, but i when scan the com by using norton anti, it doesnt shows any infection at all. So i decided to reformat my computer! But after reformatted everything, it doesnt seems to get better..it still the same lag as ever. And the worst thing is.. if i download some software from internet, it will promt that file is unable to complete due to win32 problems. How can i solve all this problems. Please help!