In case it helps... the humble PC I use to mine alt coins has an Athlon 64 X2 4000+ (2100 MHz), 6 GB DDR2 666 and a rather slow Samsung HD160JJ 160 GB SATA II HD, running Windows 7Garbage Ultimate 64 (my main PC runs the last decent OS MicroShit produced, XP 64). I just made a test - I copied my Bitcoin data directory (which I updated about a week ago) from my main PC to the mining PC and it was able to sync Bitcoin-Qt 0.8.5 without any problems - and scrypt mining at the same time.

I'm no expert on the blockchain structure, but I was wondering if maybe some parts you've already downloaded haven't become corrupted. Have you tried running Bitcoin-Qt (or bitcoind) with the -reindex and/or the -checkblocks=0 parameters? (That will take a LONG time, be warned!)

Your best bet would really be to download the whole blockchain using the current official blockchain torrent and then import it following the instructions on that thread.