Unfortunately, if you talk to Janus and get the quest to find the cat before entering the castle by invitation the quest will be broken when you do enter. The only options are to reload a save before you first talked to Janus (if available) or start over.
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If you have to start a new game, it is possible to bypass the catacombs by cheating, or simply leave Aleroth without re-doing it (though combat may be a little tough until you level up a bit; see the spoiler below for a way to level quickly at the start).
After you run around town to heal the soldiers, loot, get the first teleporter stone, etc, you can save and then use the save editor to give yourself enough experience points to get up to level 7 or 8 (just before Thelyron I had 192979 experience points), use an item editor to recreate your equipment and then teleport to the bottom of the catacombs to finish that main plot quest.
iZakaroN's SaveEditor and DAD's item editor Divine Divinity Item Editor - not as full featured, but should work fine in Vista/7 (DAD's editor requires Visual Basic runtime files which may not be installed in Vista/7 and are not included in those versions of Windows by default)
Teleporting Anywhere, via hex editor .
FWIW, I played the demo before the game was released, and wasn't thrilled about having to redo the catacombs (demo saves were not compatible with the full game). However, there was a time gap between the demo and game releases, and having a better strategy from the start helped things go faster the second time through.
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. Spoiler
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Without leaving Aleroth, you can reach level 4 before you enter the catacombs.
Explore / loot around Aleroth and kill any rats, chickens or cows that you want (50 exp on level 1, plus meat which heals a small amount), but you may want to delay completing any quests. At level 1, if you defeat the zombie Jake (the graveyard would be easiest), you will get enough experience points to bring you up to level 3. This requires a lot of hit and run with a bow, or a moderate amount using Meteorstrike. Defeat him a second time (in his cellar in his house nearby) and you get to level 4.
