Itunes says that my iphone is already synced with with a different library

The iPhone's backup maintained by iTunes does not include any iTunes content and it does not include any photos transferred from your computer to your iPhone.

In regards to iTunes content, an iPhone can be synced or manually managed with an iTunes library on a single computer only, and photos can be transferred from a single computer only. When transferring iTunes content and photos from another computer, all iTunes content and photos transferred from a different computer will be erased from your iPhone first, but your entire iPhone is not erased when doing so.

After reformatting your computer's hard drive, your computer is detected as a new or different computer by your iPhone.

Did you include your iTunes library with your computer's backup along with all other important data before reformatting the hard drive? If not, you should have. The iPhone is not designed or intended to be used as a backup storage device.

iTunes includes an option to transfer iTunes content that was purchased/downloaded from the iTunes store from an iPod or iPhone, but this is for iTunes content that was purchased/downloaded from the iTunes store only.

First you need to re-authorize your computer with your iTunes account with iTunes if you haven't already done so. With your iPhone connected and without syncing, at the iTunes menu bar go to File and select Transfer Purchases From - the name of your iPhone.

Although contacts are included with your iPhone's backup, contacts are designed to be synced with a supported application on your computer. With Windoze, this can be with Outlook 2003 or 2007 along with syncing calendar events, or with Windows Contacts for syncing contacts only - the address book used by Outlook Express or Windows Mail.

If you don't have Outlook 2003 or 2007 available and you don't use Windows Contacts for storing contact info on your computer, before syncing contacts enter one contact in Windows Contacts. Make this contact up if needed, which can be deleted later. This will provide a merge prompt with the first sync for this data, which you want to select. Syncing contact with Windows Contacts is selected under the Info tab for your iPhone sync preferences with iTunes. This way you will have all contact info available on your computer also which should be included with your computer's backup in the event your iPhone is lost or stolen.

You're welcome.

ohwell i have learnt from my mistake and next time i will make a backup of the iphone and just copy it back in once a reformat is done or if i buy a new PC.

Having your iPhone's backup available and restoring it from your computer's backup after reformatting your computer's hard drive has no effect on this and won't change it.

Copied from my previous post.

+In regards to iTunes content, an iPhone can be synced or manually managed with an iTunes library on a single computer only, and photos can be transferred from a single computer only. When transferring iTunes content and photos from another computer, all iTunes content and photos transferred from a different computer will be erased from your iPhone first, but your entire iPhone is not erased when doing so.+

+After reformatting your computer's hard drive, your computer is detected as a new or different computer by your iPhone.+

thinking about it, where is the backup for the iphone located and is it as simple as copying it into an external hard drive and then copying it back over when its ready?

This link provides the data included with your iPhone's backup along with the location of the backup on your computer's hard drive but again, having access to your iPhone's backup would not have changed anything in this situation.

http://support.apple.com/kb/HT1766