Know exactly what you mean!!!!
Did you uninstall nav before the reformat? because if you didn't, you will have erased any recollection of subcription dates and will go back to "0" date as if it was a fresh install!
I had the same problem when my pc crashed and was advised to reformat and reinstall the o/s, and when I reinstalled nav oem 2002, my subs was showing expiry date some 6 months to early!!!!!
Spent weeks trying to contact synmantec to no avail and was always directed to the USA page on this subject for the faqs, which was of no use what-so-ever!!!
In the end, I had to resubscribe for a fresh year,even though I still had 6 months to go!
Using win xp home ed sp1 1GB sdram 180HDD Intel 1.8GHz p4 Nvidia graphics Nav 2002oem version