I downloded the USB installation thing that Ubuntu released in 2012. Now I'm installing it and when it's 99% complete it shows an error like 'some file not found'
Redownload the file and try again. Use the same method as last time to get the file on the USB. If it doesn't work, try a lower version of the linux distro your using.
The file your downloading may be corrupted, so try a different version or a different download location, if the above doesn't work.