XCode and iOS SDK installation failed

thanks for your help,

i still get error

i will tell you what i did:

1- i donwnloaded it

2- then i click on the xcode 3.2.5_and_ios_sdk_4.2final.dmg then i get messege opeinging and verfiying then i got error messege in reason : invalid checksum

3- when i see this error i opened it by skip this step then i got new windows and have 2 files there when i click on step file last i got installation failed


please tell me what i can to do to fix it i know my problem in invalid checksum, how to fix it