In Upgrade.log file generated after running upgrade by clicking "Begin Upgrade" in central administration of MOSS, I found following errors in log when upgrading from SharePoint 2003 to MOSS 2007.
"[ProfileDatabaseSequence] [ERROR] [12/21/2008 3:07:31 AM]: Action 188.8.131.52 of Microsoft.SharePoint.Portal.Upgrade.ProfileDatabaseSequence failed.
[ProfileDatabaseSequence] [ERROR] [12/21/2008 3:07:31 AM]: The statistics 'SectionName' is dependent on column 'SectionName'.
ALTER TABLE DROP COLUMN SectionName failed because one or more objects access this column."
I used following query to find table having column "SectionName" in the database, since i didnt know which table had it:
SELECT name FROM sysobjects WHERE id IN ( SELECT id FROM syscolumns WHERE name = 'SectionName')
It returned "ProfileUI" table name.
I expanded the "ProfileUI" table in SPS2003's Profile DB and then explanded "Statistics". There was the dependency caused error. Deleting the SectionName from Statistics of table removed its dependency.
Now upgrade would run the "alter table drop column" command. It would have failed if I had dropped the column, so I didn't drop column manually, just removed the "SectionName" dependency.
I was able to run upgrade successfully after cleaning up and reinstalling MOSS. All the sites are displayed for me to be upgraded one by one.