that's huge! Just checked mine, it's 693M
I suggest you backup your wallet.dat, remove everything, and then redownload blockchain. I don't think it'll work if you just delete random files.
I'll be happy to know what files there are and their sizes, it shouldn't ever reach 18G, maybe we can identify its reason
The tail end of my debug.log looks like this:
InvalidChainFound: invalid block=414a52d34567fb31bd46 height=236098 trust=334503510004688 date=04/10/14 21:31:08
InvalidChainFound: current best=8b4588eebf43692e4818 height=304122 trust=312398834893033 date=04/30/14 14:01:53
ERROR: SetBestChain() : Reorganize failed
ERROR: AcceptBlock() : AddToBlockIndex failed
ERROR: ProcessBlock() : AcceptBlock FAILED
received block eccf826887c103dc7df9
Postponing 10419 reconnects
REORGANIZE
*****REORGANIZE: Disconnect 84120 blocks; 76f0e51e12fa1dedecef..8b4588eebf43692e4818
REORGANIZE: Connect 5678 blocks; 76f0e51e12fa1dedecef..65d8640176c39de6053c*****
I think you are using 1.6 or 1.7. Probably it is 1.6, or 1.7 with a messed up database.
Disconnecting 84k blocks doesn't seem logical to me, I think you're either using 1.6 or 1.7 and your wallet switches back and forth on POW+POS blockchain (1.6-) and the POS-only blockchain (1.7+)
I suggest you to update to latest version, latest versions contain latest blockchain checkpoints so your client won't try to reorganize that deep. I also suggest backup your wallet.dat, delete everything, and sync from scratch just in case, and a "checkwallet" to be sure everything's ok.