unable to reinstall Windows XP

I was unable to reinstall Windows XP after infected with trojans. The option to format is not a solution as some valuable emails have not been backed up. I attempted to reinstall windows in a separate folder, but got an error message, ‘the file ASMS on Windows XP Home SP1 CD is needed. Type the path where the file is located.’ Where can I find this file please? This is a Dell Dimension 2400 PC, the file is not on any Dell disk. Please help.