hi m8 do you just get a blank desktop or dose it not get that far ? have you tryed safemode ?
to access safemode keep taping the F8 key when you turn on the computer till you get a boot menu
from there first try last knowen good configeration if that dont work go back to the boot menu go to safemode and try using system restore to revert any changes or file coruption that is stoping your computer from starting if non of that works do you have a xp pro cd if you do put it in the cd drive and start the computer from it ou will have to do a repair instal of xp all info for this can be found here
http://www.michaelstevenstech.com/XPrepairinstall.htmandy