Can you boot in safe mode? Hold down shift from startup until you are in login screen, then release (it will take longer than normal boot, but don't release shift until you see "safe boot" on screen)
if you can, does it freeze there too?
When did it start to happen?
How full is the hard drive? If it's > 90 % full that could be one reason for the problems above. (You should always have at least 15 % of hard drive space free)
Can you boot to the install discs (of that Mac)? Does it freeze there too?
If you have the discs available that came with the Mac, running Apple Hardware Test would be one step to take. It's if I remember correct on disc 2. With disc in, boot with alt, select the hardware test, and run in looped mode for at least an hour (ctrl-L to get loop on, then select extended test. same keyboard combo to unloop after - main thing is getting more than one round of tests)
If the time and date go back to 1970, your internal battery is empty. They can be set from system preferences to current, but after restarts they may go to zero again (zero unix time started 1/1/1970).
But let's see if we can first get the freezes back to under control (or at least get a clear result from hardware test) as that's less important an issue for now.
