You can take a backup of the BOINC data directory, path printed in BOINC eventlog. Upon system restore, install BOINC but do not attach to WCG. Better, before installing, take the comp offline. Upon Installing, Exit BOINC via the manager menu, ticking the field to stop the client, not the box to remember. Once done, restore the data directory. Start BOINC and confirm your tasks are back. Only after confirming connect the internet again. Work will continue where it left off.

Alternate, before doing system restore suspend all ready to start, finish tasks running. Set client not to fetch new work and abort all suspended tasks. Hit update several times to tell server the jobs were aborted so they can be issued to other crunchers. If you dont, they will stay assigned to your comp until they expire.