blue screen on snow leopard

Sounds like it's stopping before showing the desktop. Things to try.

1) Restart and immediately hold down the Command+Option+P+R keys. Keep holding them through two or three startup chimes. Let go of the keys and let the Mac start normally.

2) Restart and immediately hold down the Shift key to boot into Safe Mode. If it works, a progress bar will appear towards the bottom center of the screen. You can let go of the key. Once at the desktop, restart again normally.

One other thing you could try before heading out. Put the Snow Leopard DVD in the drive. Restart and hold down the C key to boot to the DVD. Once it's up, launch Disk Utility. In DU's interface, select the hard drive and click Repair Disk. If any errors are found, repeat until it shows no errors. Restart normally.

If DU does find errors and they cannot be fixed in three tries, then you would need to move onto more capable software, such as DiskWarrior.