First problem with blockchain corruption : delete blkindex.dat and blk0001.dat and re-download.
Second problem with GUIMiner shutdown : Disable Microsoft Security Essentials Behavior Detection. (found on another thread in this forum).

Having the wallet as a file on your computer? I agree it's P2P so there can't be a master server keeping track or user accounts, but I though it was stored on the "cloud" somewhere. There will be thousand of people loosing all their money during HD crashes and stuff. How does that not unbalance the market? Very bad idea.

With my rig (Core i5-2500K @ 4.4ghz and Nvidia GTX 560 Ti), I get about 50Mhash/s. Looking at my pool result, I barely got 0.001BTC, so about half a cent in one night. Let's say I make 1-2cent per day, it won't get interesting any time soon. I'm not paying for electricity here, but hardware usage might become a long-term problem, and it cost a lot more than this bitcoin mining will gives back.