@ Mithun
1.Take the backup of your downloaded data (it generally shows full size even if it is partially downloaded, incase of single files) and the main torrent file in some external media.
2. Format, Install do whatever you want with you computer. Now i assume you have a reinstalled OS and your client is utorrent
3. Now copy back the partially downloaded file / file that you want to seed to any folder you like in your machine.
4. Fire up utorrent and add the torrent that you want to continue downloading / seeding and when it asks where to save, give the location of the folder where you just copied the backup.
5. It will start a recheck of the contents and the percentage bar will increase till a point that you downloaded in previous instance (sometimes this may vary by some points, not to worry) and then it will continue to download here. In case of seeding it will go till 100% and then start seeding.
PS: Ensure that you properly point the torrent to correct folder when you want to restart the downloading/seeding.