Not enough information here to make an educated guess. Pro Tools is freezing is an incredibly broad brush stroke.
Does it lock up to the point of needing to re-boot the computer? Does it lock up forcing you to re-boot Pro Tools? Does it throw an error code? Does it behave this way on all sessions?
It just freezes and says PT is not responding and I have to go to task manager and shut it down.
I have 3 gig ram, 2g cpu, 120 gig HD...oh and crappy vista, Its a laptop and will not let me install xp
If you are running Vista, then make sure that you have the latest updates from both Microsoft and Digidesign.
Locking up on play can point to a number of things. Too many plugins in your tracks, or too many CPU intensive plugins (as in reverbs etc). Maybe you have not set your CPU / memory usage correctly? If you have a large or heavy session, set your RTAS memory usage to the highest (1024).
If this doesn't help, and you are running a genuine version of Vista & your Digidesign software, then I would check on the Digidesign website for compatibility of your laptop and Pro Tools.