If it happens in Safe Mode, then that tends to point to either a horribly corrupted OS, or a hardware issue.
So, first test, is to reboot the system with your 10.5 install DVD in the drive while holding down the "D" key. This will bring you to a simple hardware test program. Be sure to tick the extended test box. See if that finds anything. Even if it doesn't, it doesn't necessarily rule out hardware, but it makes some kind of OS update gone horribly wrong more likely.
If the hardware test comes up fine, I'd suggest trying an archive and reinstall. This takes your existing system, dumps the whole thing into a folder, and then installs a new copy of the OS. For a day or two, keep it to just the essentials, don't even install any updates, and see if the problem persists. If it does, I'd say your hard drive is on its way out. If it behaves, then start by installing just the updates from Software Update, and give it another day or two to see if things remain stable. If they do, you can slowly start adding in the rest of your programs to try and find the problem child. It might even end up being that your current problem is a fluke occurrence, and a fresh install of the OS is all you need.
If you do find some hardware problem, if it's anything other than bad RAM (highly unlikely) or a bad hard drive, and the system is not under warranty... Start looking for a new laptop. If it's not the RAM or hard drive it'd be the logic board, and that will run you probably at least $600 to get replaced. Considering new 13" MacBook Pro's go for around $1200 or less, it's not that economical to repair. And if you do get a new system, be sure to pick up the AppleCare plan for it. Could be the best $250 you ever spent. Anything other than the hard drive or RAM, which you could replace yourself, will likely cost you well over $250 to repair.
In the past week, my computer has been Beachballing/pinwheeling (whatever you wish to call it) when ever i run programs. im pretty sure since the last update. It seems to freeze up mostly when using internet related programs. ie. firefox safari or Instant messengers. it doesn't seem to have as much a problem using regular computer programs, but its still there none the less, just not as bad.
Ive tried repairing disk, removing addons, removing items from my start up. I'm a fairly new Mac user so i'm at a loss to what to try to do to try to fix things. even in safe mode it still locks up. it can be locked up anywhere from a few seconds - when ever i have to sadly force quit or restart the computer by *winces* holding down the power button.. >.<
Mac ox 10.5.8
2.53 GHx intel core 2 duo