[classicheepfan]

My wife was editing some pictures and the machine froze. She re-started it and now it won't go past the login screen. It allows you to enter your username and password, but it won't finish the login process. Does anyone have any ideas on how to fix this?

[osxpro]

Try to get your hands on a copy of Disk Warrior. DW is the best tool I've ever used for repairing systems. If you do not have DW, you can try and boot the system into Single User Mode. Once in SUM you'll want to type at the prompt:
sudo /sbin/fsck -fy >then hit return
That command will run a disk check and repair and corrupt directories. If the disk check program finds any issues, it will attempt a repair. If any issues are found and fixed, you will want to run the command again until there are not issues found. When that has completed, you will want to run the next command.
sudo diskutil repairpermissions /

Once you get your system up and running, I HIGHLY recommend downloading "Applejack". Applejack is a small program you install that allows you to run several utilities in SUM (Single User Mode). The advantage of Applejack is that you don't have to enter complicated unix commands. Instead, you are presented with a list of numbers, 1-5. When you type the number, it will run the script for you without having to type the entire unix command.
1. Will run the fsck disk check
2. Will run repair permissions
3. Will clean out user caches
4.W ill check you preferences for any corrupt ones
5. Will clean your core temp files.

Once a week I run diskwarrior then use the Applejack app in SUM. This simply maintenance routine will keep your disk in good health. Because I run this routine, I've never had any major issues with any of my 3 Mac's.

BTW, don't forget to backup your system with Time Machine!!!!!!!!!!
If you need anymore assistance, please send me an email - I would be happy to help a fellow Mac user.

[Cold Warrior]

before buying Disk Warrior, just try your OS X DVD that came with the Mac.

Boot from that DVD and run Disk Utility > Repair Disk. Resolve startup issues and perform disk maintenance with Disk Utility and fsck
That link also lists several more options (like fsck from single-user mode, which is also something worth trying even before booting from the DVD).

Boot the Apple Hardware Test (AHT) to test for bad hardware. Intel-based Macs: Using Apple Hardware Test

If that doesn't help, and Disk Utility didn't find any unrepairable errors, you could try to reinstall your OS.

If things continue to malfunction, your hard drive or RAM could be failing in a way that AHT didn't detect.