:confused: My guess would be the output on your SM58 is higher than that of "normal" PC mics... closer to what the line-level input on your MBP is expecting.
I haven't tried an XLR->1/8" adapter with my Sennheiser HMD280 Pro mic yet but I've never had luck with any other headsets or normal PC...
All current Macs lack a mic preamp, they only have line-level inputs.
So, you'll either need a mic preamp (Griffin iMic would work, it's USB), or a USB mic. The Blue snowflake would be an excellent USB mic choice, but is probably over budget.
Open your c:\windows\system32\drivers\etc\hosts file in notepad and delete anything directing myspace to google's IP address. Sounds like someone is having some fun with you.