Recently, my music started skipping in iTunes, and the computer was starting to run slow. It couldn't shut down properly, so I had to force it. After that, the computer would stay on the gray screen for a really long time before going to the blue screen and freezing, not even showing the cursor. I tried fsck, disk utility, moving the preference files in single user mode, zapping the PRAM, starting in safe mode, and resetting the PMU. All pointed to "Macintosh HD is OK." I also tried watching the startup in Verbose Mode, and there were no problems until the CLI went away and stayed on the blue screen again. The only significant change I've made before this happening was adding a new account for guests. Could that be the cause?