How To Eliminate A Deadly Computer Virus?

i have windows xp sp2 installed in my computer and i forgot to install an internet security suite..when the computer started to perform slow i installed kaspersky internet security and it detected some sort of trojan.gamethief.win32, but couldnt block or delete the virus… so there was nothing i could do and about 3 days later the computer wont start(the computer starts upto windows logo and restarts) so i entered in safe mode..i had a 500GB hard drive(not partitioned) and all my games and stuff were in it… so i wanted to install windows xp again in the unpartitioned space without formatting the pc so i downloaded diskimaging software known as acronis trueimage home and tried to install it in safe mode but i couldnt…it said that the software may not be able to be installed while in safe mode so i tried another diskimaging software norton ghost and i came upto the same problem…so i would like to know how to install a new fresh version of windows without formatting my harddrive??