First of all, to back up your wallet, assuming you're using the standard client, find the wallet.dat file in your bitcoin directory (location depends on operating system) and copy it to a safe location. Given the amount of money you've got, you really should be backing up your wallet at regular intervals anyway, even if you don't plan on reinstalling anything, in case your hard drive goes kaput and you lose everything or something else bad happens.
However, you probably don't need to reinstall bitcoin. It sounds like your copy of the blockchain may be corrupt. Assuming you're using the standard bitcoin client, close bitcoin, backup the files blk0001.dat and blkindex.dat in your bitcoin directory to another directory (note that you'll need a spare 1.5 GB of disk space), delete the originals, then restart bitcoin. This will cause it to download the whole blockchain again from scratch.
If it can't download the blockchain again at all, then you've got a network problem, and you should copy the blk0001.dat and blkindex.dat files back (after closing bitcoin) so that you don't have to download the blockchain again after you fix it.
If it is able to download the blockchain again without any problems, see if it can get past the troublesome block this time around and update successfully. If so, then the problem was that your original copy of the blockchain was corrupt, and you can delete your backup of the corrupt files, because there's no point keeping them around.