We had one issue of a dotnetnuke site gone stinkin mad with the MSSQL8 SQL 2000 to 2008 upgrade.
here is a fix, it will clear out logs and schedulehistory, but solve cpu usage problems, SQL query:
use user_DBNAME
TRUNCATE TABLE “SiteLog”
TRUNCATE TABLE “EventLog”
TRUNCATE TABLE “ScheduleHistory”
Then reindex and update stats, should be in good order then.