The computer worked fine before. Now when I sign in at the login screen the computer won't go past the gray screen with the Apple logo and the spining circle. After a minute or so the Apple logo turns into a circle with a slash going through it sort of like "No Smoking" sign and this continues indefinatley. I've looked for help but couldn't find any, I've also tried holding "Shift" after the boot up chime but it didn't seem to work as nothing happened and it took me to the login screen once again. Im using the Guest account and it only gives me access to Safari
See this Apple note on that startup issue. Where it talks about booting from an OS X disk to run Disk Utility, you should be able to boot to the recovery partition (boot holding command and r keys down). If that doesn't work, and you upgraded to 10.7 Lion, then use an OS X DVD to boot and run Disk Utility.