Having trouble restoring backup of my iPhone
The simplest thing to do is to take your iPhone to a new computer and see if you can determine if the problem is the iPhone / OS on that device or the problem lies with your computer.

Also - you might check iTunes to be sure you have the old backup stored safely (perhaps backing up the computer itself) 

iTunes places the backup files in the following places:


Mac: ~/Library/Application Support/MobileSync/Backup/  
Windows XP: \Documents and Settings(username)\Application Data\Apple  Computer\MobileSync\Backup\  
Windows Vista and Windows 7: \Users(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\ 


Once you know which of the two parties is not working correctly, you should be able to take your troubleshooting to the next step and consult the site here or Apple's support articles for how to proceed:


http://www.apple.com/support/iphone/assistant/restore/
http://www.apple.com/support/iphone/assistant/itunes/