It sounds like the blockchain database is corrupted. Did Bitcoin ever work?
The user data directory is %APPDATA%/Bitcoin (you can type that in to windows explorer or a search to bring up that folder.) The blockchain is the two files blk0001.dat and blkindex.dat in that directory. You can delete these two files and all the log files in the "database" subdirectory, and then re-launch Bitcoin, which will make it begin downloading all the blocks again.
You must also be able to read and write data to this directory. This is a user directory, so permissions to save program data should be not an issue.
You can refer to this thread
for more details about the blockchain and downloading it.