Depends where the HDD data is.
If its stored in the current Windows "user" folder like My Docs then when you do a new install Windows creates a file called windows old and everything in the current windows folder is saved there.
You could create a new partition and then use the Windows Easy Transfer tool
this will allow you to specify what to back up and where.
Or as has been mentioned you could back up to an external device, depending on size of backup, Flash drives, HD or DVD etc, I've even done backups to 32GB cards on my HTC phone for people when really stuck and nothing else to hand.