Entropy causes hard disks to go bad.
When it freezes, can you hear repetitive motion from the disk, a result of seeking the same area repeatedly? That could be a sign of impending failure.
Hard disks are not terribly difficult to replace.
If the cursor itself cannot be made to move at all, it's a different manifestation of entropy.
General purpose Mac troubleshooting guide: Isolating issues in Mac OS X
Creating a temporary user to isolate user-specific problems: Isolating an issue by using another user account
Memory limitations: Using Activity Monitor to read System Memory and determine how much RAM is being used
Identifying resource hogs and other tips: Runaway applications can shorten battery runtime
Starting the computer in "safe mode": Mac OS X: What is Safe Boot, Safe Mode?
To identify potential hardware problems: Apple Hardware Test