Cpuminer problem: empty reply from server
The miner you are using does not support the stratum protocol. Instead you must use a pool that uses the old and now obsolete getwork protocol

If I see it correctly BTCGuild still offers the getwork access with the following URL:

minerd.exe -o http://btcguild.com:8332 -u username_1 -p password -a sha256d -R 2