If the data has a substantial financial value to you, turn off the computer, remove the hard disk and send it to a data recovery specialist.
If you want to recover the data yourself, your first priority is to make a backup of the disk before more data loss occurs, every minute that you have Windows running there is a chance that more of your old data is being overwritten by files being created or extended (e.g. event logs) in the new partition. Boot the PC using a recovery CD/DVD and make an image backup of the disk. You may need to purchase an additional disk to hold the backup.
If the old data partition was not at the start of the disk, there is probably more likelihood that it has not been much overwritten by the new windows installation. You'll need to Purchase data recovery software (or locate free software) that will scan the disk for data sectors and recreate the old files.
I can't recommend any particular software as this has never happened to me - I always backup my important data before installing a new operating system.