Yes, very brief scan on bootup sounds like the cache

The only way to clear it would be to delete the file C:\Documents and Settings\All Users\Application Data\Spybot - Search & Destroy\ProcCache.sbc, or download an update (the cache is completely reset when loading again after an update, since a new update might recognize bad file that wasn't recognized before the update).

My tests were usually around 25 vs 100 MB. Not sure if that included the latest Beta.sbi, if not, that would probably explain the difference, the next update (available as beta since last Wednesday) includes a huge jump