Reinstall windows 7

Yes, you can but not by adding the old tasks to the latest ones.  If you look at your computers list you will see both versions of your computers listed there as two separate host IDs.  You can't transfer tasks allocated to one ID to the other ID.  You would corrupt the project's database if you could.

The correct way to have done your upgrade would be to have completed all the outstanding tasks and have returned them first.  Then uninstall BOINC and save a copy of the complete BOINC tree that remains in case something goes wrong with the upgrade.  Even if you have to reinstall Windows, you could then copy back the BOINC tree so that it will be found when you come to reinstall BOINC.  I don't use Windows so I can't give you detailed steps.  I can tell you what I think, but if you are in any doubt, please seek independent advice.

If you still have the old BOINC tree available on the old disk, you can still recover those tasks if you are careful (and if the tasks haven't expired).  I do this type of transfer quite regularly in Linux and it's very simple and straightforward there.  If you decide to do it in Windows, it is at your own responsibility.  The steps would go something like this:-

1. In your current installation, set No New Tasks (NNT) and allow all current tasks to complete and return.
2. Uninstall BOINC and remove the remaining 'no tasks' BOINC tree to external media - a backup just for safety.
3. Copy the complete BOINC tree from the old disk to replace the 'no tasks' BOINC you have just backed up.
4. Reinstall BOINC, making sure the old BOINC tree (with all its tasks) is found during the installation.
5. Start crunching, and your old tasks should restart from saved checkpoints (if any) that existed prior to the upgrade.
6. Once crunching starts and work is returned, BOINC will request further work as needed.
7. If all goes well, you can throw away your backup copy of the 'no tasks BOINC tree that you saved.

An advantage of recovering your old tasks this way is that you also recover your credit history and creation date for that host.  Your new host ID will become dormant and will slip off the current hosts list in 30 days time.

Note 1:  It's your responsibility to know what all bits of the "complete BOINC tree" are and where they go.  If in any doubt, ask for advice from a Windows 'expert' - that's certainly not me :-).

Note 2:  The above assumes that Einstein is your only active project.  If you have other active projects as well, it's still doable but it's a different and more complicated technique.