In addition to what "osxx" mentioned above...the reason why you're seeing so many "beachballs" is most likely because:
- too many applications are open at once
- don't have enough ram relative to the # of simultaneous apps you have open
- the computer is not being restarted enough (this clears the ram)
Whenever you start seeing "beachballs":
- open "Activity Monitor" (Applications > Utility Folder)
- click on the "System Memory" tab
- take a look at the "page outs" and "swap used"
If these numbers are large & getting larger...this is why things are slowing down. "Page Outs" and the "Swap used" are indicators of how much info is being written to the hard drive because there's not enough ram to hold all the info. Writing info to the hard drive is EXTREMELY slow compared to the speed of ram.
In the ideal situation...you would want the "page out" and "swap used" values to be zero! I've upgraded the ram on my wife's MacBook, my MacBook Pro, and my Mac Pro...and all three now regularly have zero "page outs" and zero "swap used".
I also monitor these values regularly...and when they are no longer zero (maybe after a couple weeks of not restarting the computer)...I restart the computer and everything is back to zero. This restart...acts as a "brain dump" so to speak.
If someone has too many applications open at once, doesn't have enough ram, or doesn't restart their computer regularly...the "page outs" and "swap file" values start getting real high/real large...and this is when things slow down...and frequent "beach balls" start appearing.
Here are some articles on the topic as well:
Does my mac need more memory?
Mac OS X: Reading system memory usage in Activity Monitor
Does your Mac need more memory? How to know if you need a RAM upgrade
HTH,
- Nick
p.s. If nothing above works...how full is the hard drive (gigabytes used vs. gigabytes free)...and lastly...there is the possibility that the HD is failing.
p.p.s. Also try "Repairing Permissions" within "Disk Utility".