Error Installation of Service pack

Currently i having a Windows XP Pro, lets name as PC_A (just reformatted), cuz of company firewall issue, i cant update from internet using that particular PC, then i download Windows service Pack manually, the service pack work smoothly with other PCs, but when i try to install to PC_A, it prompt out an error
“File is corrupt”, i thought was the installer problem, but when i use back the same installer to other PC, is work, now i just wonder what will cause the error prompt out